Fix windows emsdk returncode (#1623)

When `emsdk install foo` would fail on Windows, the return code would
get ignored since there were later commands in the script file.

To fix that, capture the error code from the emsdk call, and output that
as the final return code.

Also a drive-by Python path fix, which seemed to be out of date in the
.ps1 file.

emsdk.bat

@@ -35,6 +35,13 @@ set EMSDK_PY=python
 :end
 call %EMSDK_PY% "%~dp0\emsdk.py" %*
 
+:: If emsdk returned with error code >= 1, then skip executing emsdk_set_env
+:: below.
+if ERRORLEVEL 1 (
+  endlocal
+  exit /b %ERRORLEVEL%
+)
+
 endlocal
 
 :: python is not able to set environment variables to the parent calling