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 <vbendeb@chromium.org>
Change-Id: Ie9f41110becb67936f49095ff36a8eeaa7a4f114
Reviewed-on: https://chromium-review.googlesource.com/1058722Reviewed-by: Aaron Gable <agable@chromium.org>