Handle exceptions in check_gn_headers.py

If exceptions are thrown, the main process would wait forever if they
are not handled. This could potentially cause timeout on the bots.

BUG=661774

Review-Url: https://codereview.chromium.org/2903063006
Cr-Commit-Position: refs/heads/master@{#474873}
1 file changed