print access errors separately
print access errors separately

When user invoking 'my_activity.py' does not have an account on one or
more of the servers the utility queries, access errors are reported
interleaving with access progress report printouts, this creates a
messy looking output.

With this patch access error messages are accumulated in a set and
then, if there are any, get printed out separately.

TEST=verified that error messages are printed neatly after servers
     queries finished running:

  > my_activity.py -u xxxyyzz
  Loading data.................................

  Access Errors:
  Looking up 'pdfium-review.googlesource.com': Bad Request: User xxxyyzz not found
  Looking up 'android-review.googlesource.com': Bad Request: User xxxyyzz not found
  Looking up 'chrome-internal-review.googlesource.com': Bad Request: User xxxyyzz not found
  Looking up 'chromium-review.googlesource.com': Bad Request: User xxxyyzz not found

Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Change-Id: Ie9f41110becb67936f49095ff36a8eeaa7a4f114
Reviewed-on: https://chromium-review.googlesource.com/1058722
Reviewed-by: Aaron Gable <bbed39beedae4cdb499af742d2fcd6c63934d93b@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools