Error on downloading a 64-bit package on a non-64-bit OS (#265)

Also fix the can_be_installed which was always broken it seems - it returns True for success or a string for error, but didn't check if the output is True.

Includes a test, which hacks up emsdk to make it think it's on 32-bit, and verifies the error and message.

test.py

@@ -1,11 +1,26 @@
 import os
 import subprocess
+import sys
 
 # Utilities
 
 def check_call(cmd):
   subprocess.check_call(cmd.split(' '))
 
+def failing_call_with_output(cmd, expected):
+  proc = subprocess.Popen(cmd.split(' '), stdout=subprocess.PIPE)
+  stdout, stderr = proc.communicate()
+  assert proc.returncode, 'call must have failed'
+  assert expected in stdout, 'call did not have the right output'
+
+def hack_emsdk(marker, replacement):
+  src = open('emsdk').read()
+  assert marker in src
+  src = src.replace(marker, replacement)
+  name = '__test_emsdk'
+  open(name, 'w').write(src)
+  return name
+
 # Set up
 
 open('hello_world.cpp', 'w').write('int main() {}')
@@ -57,3 +72,8 @@ check_call('./emsdk activate sdk-tag-1.38.33-64bit')
 
 print('test binaryen source build')
 check_call('./emsdk install --build=Release binaryen-master-64bit')
+
+print('test 32-bit error')
+
+failing_call_with_output('python %s install latest' % hack_emsdk('not is_os_64bit()', 'True'), 'this tool is only provided for 64-bit OSes')
+