[infra] Error, don't crash setup_toolchain.py

In case the environment is incorrectly set on Windows (e.g.
GYP_MSVS_OVERRIDE_PATH pointing to a non-existing path), the
setup_toolchain.py script may raise an exception.

The script attempts to run a subprocess to gather the environment
variables. But it does not check that the process actually
succeeded.
In case the process fails the environment data will be invalid and
the script will ultimately raise an exception.

This change adds a check for the process failing, and gracefully
warns and exits the script instead.

TEST=Manual build on Windows.
Change-Id: I3624bdf98e0ebe48b403825a90ce2e86c2ed8008
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/242860
Commit-Queue: Clement Skau <[email protected]>
Reviewed-by: Tess Strickland <[email protected]>

Author: Clement Skau

build/toolchain/win/setup_toolchain.py

@@ -118,8 +118,15 @@ def main():
     args.extend(('&&', 'set'))
     popen = subprocess.Popen(
         args, shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
-    variables, _ = popen.communicate()
-    env = _ExtractImportantEnvironment(variables)
+    stdout_data, stderr_data = popen.communicate()
+    if popen.returncode != 0:
+      print('Error, got returncode:', popen.returncode)
+      print('## stdout:')
+      print(stdout_data)
+      print('## stderr:')
+      print(stderr_data)
+      sys.exit(2)
+    env = _ExtractImportantEnvironment(stdout_data)
     env['PATH'] = runtime_dirs + ';' + env['PATH']
 
     if cpu == target_cpu: