Error checking to clarify SYSTEMROOT errors

build/toolchain/win/setup_toolchain.py

Index: build/toolchain/win/setup_toolchain.py
diff --git a/build/toolchain/win/setup_toolchain.py b/build/toolchain/win/setup_toolchain.py
index f395044adaec947dda459528566d231ef12a4fa0..963f00230b07204aaed4ff3fc7ee16db6d44b63d 100644
--- a/build/toolchain/win/setup_toolchain.py
+++ b/build/toolchain/win/setup_toolchain.py
@@ -32,6 +32,10 @@ def _ExtractImportantEnvironment(output_of_set):
       'tmp',
       )
   env = {}
+  # This occasionally happens and leads to misleading SYSTEMROOT error messages
+  # if not caught here.
+  if output_of_set.count('=') == 0:
+    raise Exception('Invalid output_of_set. Value is:\n%s' % output_of_set)
   for line in output_of_set.splitlines():
     for envvar in envvars_to_save:
       if re.match(envvar + '=', line.lower()):
@@ -123,6 +127,8 @@ def main():
     popen = subprocess.Popen(
         args, shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
     variables, _ = popen.communicate()
+    if popen.returncode != 0:
+      raise Exception('"%s" failed with error %d' % (args, popen.returncode))
     env = _ExtractImportantEnvironment(variables)
     env['PATH'] = runtime_dirs + ';' + env['PATH']