If main is not fetchable, report the error to end user.

R=gavinmak@google.com

Bug: 1277136
Change-Id: I8c061719f9aadc42786045b3c5331bab3d0caae7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3389824Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>