Make bisect-builds.py exit gracefully on error.

In VerifyEndpoint(), the code that catches an exception tries to
continue, only to hit an UnboundLocalError. Instead of doing that, raise
an SystemExit to fail gracefully.

Change-Id: Idf1174ef01fbe7a086712fa6e7b9cca7ada97a69
Reviewed-on: https://chromium-review.googlesource.com/c/1313561Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Prasad Vuppalapu <prasadv@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606947}

tools/bisect-builds.py

@@ -738,6 +738,7 @@ def VerifyEndpoint(fetch, context, rev, profile, num_runs, command, try_args,
         context, rev, fetch.zip_file, profile, num_runs, command, try_args)
   except Exception, e:
     print >> sys.stderr, e
+    raise SystemExit
   if (evaluate(rev, exit_status, stdout, stderr) != expected_answer):
     print 'Unexpected result at a range boundary! Your range is not correct.'
     raise SystemExit