Reviewed by Adam Barth.

        sheriffbot can't roll out security patches
        https://bugs.webkit.org/show_bug.cgi?id=39136

        Make SheriffBot determine if it's authorized to view a bug
        whose change it wants to rollout before it tries to rollout
        the change.

        Moreover, make both webkit-patch and Sheriffbot provide human-
        readable error messages when they are not authorized to view
        a bug and when a bug number is invalid.

        Currently, Sheriffbot does not parse Bugzilla bugs for
        <bug error="...">, which indicates an error when retrieving
        a bug. In particular, error="NotPermitted" if a person (or bot)
        is not authorized to view a bug. For such error="NotPermitted" bugs,
        Sheriffbot raises an exception when parsing the bug report and
        this exception does not explicitly indicate Sheriffbot's lack
        of authorization. Instead, Sheriffbot should explicitly check
        for the presence <bug error="..."> before operating on a bug
        and error with a human-readable message if it's not permitted
        to view it.

        * Scripts/webkitpy/common/net/bugzilla.py: Added BugzillaError class.
        * Scripts/webkitpy/common/net/bugzilla_unittest.py:
          - Added unit test test_bug_parsing_for_bugzilla_not_permitted_error().
          - Added unit test test_bug_parsing_for_bugzilla_not_found_error().
          - Added unit test test_bug_parsing_for_bugzilla_invalid_bug_id_error().
        * Scripts/webkitpy/tool/bot/sheriff.py: Modified to catch BugzillaError.
        * Scripts/webkitpy/tool/commands/download.py: Ditto.
        * Scripts/webkitpy/tool/commands/queues.py: Ditto.
        * Scripts/webkitpy/tool/commands/upload.py: Ditto.
        * Scripts/webkitpy/tool/steps/closebug.py: Ditto.
        * Scripts/webkitpy/tool/steps/obsoletepatches.py: Ditto.
        * Scripts/webkitpy/tool/steps/preparechangelog.py: Ditto.
        * Scripts/webkitpy/tool/steps/updatechangelogswithreviewer.py: Ditto.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@68493 268f45cc-cd09-0410-ab3c-d52691b4dbfc