Avoid post-install when nothing is installed (#736)

This avoids re-running the post-install scripts when commands such as
`./emsdk install latest` a re-run.  This re-running of npm ci can be
significant slowdown especially during testing and developerment.

Becuase of the refactoring this change change also means we exit ealier
when a given tool fails to install.  In general we want to error out as
early as possible on the first failure so as not to bury it.

emsdk.py

@@ -1828,9 +1828,11 @@ class Tool(object):
       return None
 
   def install(self):
+    """Returns True if the Tool was installed of False if was skipped due to
+    already being installed.
+    """
     if self.can_be_installed() is not True:
-      print("The tool '" + str(self) + "' is not available due to the reason: " + self.can_be_installed())
+      exit_with_error("The tool '" + str(self) + "' is not available due to the reason: " + self.can_be_installed())
-      return False
 
     if self.id == 'sdk':
       return self.install_sdk()
@@ -1838,33 +1840,42 @@ class Tool(object):
       return self.install_tool()
 
   def install_sdk(self):
+    """Returns True if any SDK component was installed of False all componented
+    were already installed.
+    """
     print("Installing SDK '" + str(self) + "'..")
+    installed = False
 
     for tool_name in self.uses:
       tool = find_tool(tool_name)
       if tool is None:
-        print("Manifest error: No tool by name '" + tool_name + "' found! This may indicate an internal SDK error!")
+        exit_with_error("Manifest error: No tool by name '" + tool_name + "' found! This may indicate an internal SDK error!")
-      success = tool.install()
+      installed |= tool.install()
-      if not success:
+
+    if not installed:
+      print("All SDK components already installed: '" + str(self) + "'.")
       return False
+
     if getattr(self, 'custom_install_script', None) == 'emscripten_npm_install':
       # upstream tools have hardcoded paths that are not stored in emsdk_manifest.json registry
       install_path = 'upstream' if 'releases-upstream' in self.version else 'fastcomp'
-      success = emscripten_npm_install(self, os.path.join(emsdk_path(), install_path, 'emscripten'))
+      if not emscripten_npm_install(self, os.path.join(emsdk_path(), install_path, 'emscripten')):
-      if not success:
+        exit_with_error('post-install step failed: emscripten_npm_install')
-        return False
 
     print("Done installing SDK '" + str(self) + "'.")
     return True
 
   def install_tool(self):
+    """Returns True if the SDK was installed of False if was skipped due to
+    already being installed.
+    """
     # Avoid doing a redundant reinstall of the tool, if it has already been installed.
     # However all tools that are sourced directly from git branches do need to be
     # installed every time when requested, since the install step is then used to git
     # pull the tool to a newer version.
     if self.is_installed() and not hasattr(self, 'git_branch'):
       print("Skipped installing " + self.name + ", already installed.")
-      return True
+      return False
 
     print("Installing tool '" + str(self) + "'..")
     url = self.download_url()
@@ -1892,7 +1903,9 @@ class Tool(object):
       else:
         success = False
 
-    if success:
+    if not success:
+      exit_with_error("Installation failed!")
+
     if hasattr(self, 'custom_install_script'):
       if self.custom_install_script == 'emscripten_post_install':
         success = emscripten_post_install(self)
@@ -1907,6 +1920,9 @@ class Tool(object):
       else:
         raise Exception('Unknown custom_install_script command "' + self.custom_install_script + '"!')
 
+    if not success:
+      exit_with_error("Installation failed!")
+
     # Install an emscripten-version.txt file if told to, and if there is one.
     # (If this is not an actual release, but some other build, then we do not
     # write anything.)
@@ -1916,21 +1932,15 @@ class Tool(object):
       if version:
         open(emscripten_version_file_path, 'w').write('"%s"' % version)
 
-    if not success:
-      print("Installation failed!")
-      return False
-
     print("Done installing tool '" + str(self) + "'.")
 
     # Sanity check that the installation succeeded, and if so, remove unneeded
     # leftover installation files.
-    if self.is_installed(skip_version_check=True):
+    if not self.is_installed(skip_version_check=True):
+      exit_with_error("Installation of '" + str(self) + "' failed, but no error was detected. Either something went wrong with the installation, or this may indicate an internal emsdk error.")
+
     self.cleanup_temp_install_files()
     self.update_installed_version()
-    else:
-      print("Installation of '" + str(self) + "' failed, but no error was detected. Either something went wrong with the installation, or this may indicate an internal emsdk error.")
-      return False
-
     return True
 
   def cleanup_temp_install_files(self):
@@ -3030,9 +3040,7 @@ def main(args):
         tool = find_sdk(t)
       if tool is None:
         return error_on_missing_tool(t)
-      success = tool.install()
+      tool.install()
-      if not success:
-        return 1
     return 0
   elif cmd == 'uninstall':
     if not args: