rebaseline_server: better reporting for bad URL fetch

context: https://codereview.chromium.org/183763025/ ('vm_run_skia_try/write_json_summary: run skpdiff within imagediffdb, not on its own')

NOTREECHECKS=True
NOTRY=True
R=rmistry@google.com
TBR=rmistry

Author: epoger@google.com

Review URL: https://codereview.chromium.org/183833019

git-svn-id: http://skia.googlecode.com/svn/trunk@13654 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed