Add a more detailed exception error on staging failure.

If an artifact can't be found in GS, the exception should contain the URI
that was searched for the artifact. Now it does.

BUG=None
TEST=It helped me debug a local problem.

Change-Id: I382a3483e7ede94a0c78f9eb595fe10601a78e58
Reviewed-on: https://chromium-review.googlesource.com/177816
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>
1 file changed