Improve error message when a commit is not found

Improve error message when gclient sync errors due to a bad commit.

R=aravindvasudev@google.com

Fixed: 1078864
Change-Id: I574c7f7d2b5419cf96a4b3a3e3c57e16baeeecf0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3362175
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

git_cache.py

@@ -348,6 +348,7 @@ class Mirror(object):
       self.RunGit(['cat-file', '-e', needle])
       return True
     except subprocess.CalledProcessError:
+      self.print('Commit with hash "%s" not found' % revision, file=sys.stderr)
       return False
 
   def exists(self):