[tools] gn.py: Ensures args parsed.

Since parse_args() returns None in case parsing failed we should
check for this and bail.
Previously we'd still try to run RunGnOnConfiguredConfigurations
on None and crash with an unrelated issue.
This would overshadow the otherwise nice error message.

Change-Id: If9e267c8eedaaacb388fd8f8475ec0e861060ed5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/187900
Auto-Submit: Clement Skau <cskau@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>

tools/gn.py

@@ -575,13 +575,17 @@ def cleanup(command):
 
 def Main(argv):
     starttime = time.time()
+
     args = parse_args(argv)
+    if args is None:
+        return 1
 
     result = RunGnOnConfiguredConfigurations(args)
 
-    endtime = time.time()
     if args.verbose:
+        endtime = time.time()
         print("GN Time: %.3f seconds" % (endtime - starttime))
+
     return result