[ews] Improve handling of invalid changelog

[Aakash Jain]

[ews] Improve handling of invalid changelog.

In https://ews-build.webkit.org/#/builders/28/builds/2949 7  validate-changelog-and-reviewer step failed with below exception because ChangeLog was invalid (started with :wq). Such issue solved be handled in a better manner, and a user readable error message should be displayed.

Traceback (most recent call last):
  File "Tools/Scripts/webkit-patch", line 80, in <module>
    main()
  File "Tools/Scripts/webkit-patch", line 75, in main
    WebKitPatch(os.path.abspath(__file__)).main()
  File "/Volumes/Data/worker/Commit-Queue/build/Tools/Scripts/webkitpy/tool/multicommandtool.py", line 305, in main
    result = command.check_arguments_and_execute(options, args, self)
  File "/Volumes/Data/worker/Commit-Queue/build/Tools/Scripts/webkitpy/tool/multicommandtool.py", line 123, in check_arguments_and_execute
    return self.execute(options, args, tool) or 0
  File "/Volumes/Data/worker/Commit-Queue/build/Tools/Scripts/webkitpy/tool/commands/abstractsequencedcommand.py", line 55, in execute
    self._sequence.run_and_handle_errors(tool, options, state)
  File "/Volumes/Data/worker/Commit-Queue/build/Tools/Scripts/webkitpy/tool/commands/stepsequence.py", line 73, in run_and_handle_errors
    self._run(tool, options, state)
  File "/Volumes/Data/worker/Commit-Queue/build/Tools/Scripts/webkitpy/tool/commands/stepsequence.py", line 67, in _run
    step(tool, options).run(state)
  File "/Volumes/Data/worker/Commit-Queue/build/Tools/Scripts/webkitpy/tool/steps/validatereviewer.py", line 53, in run
    if changelog_entry.has_valid_reviewer():
AttributeError: 'NoneType' object has no attribute 'has_valid_reviewer'

[Aakash Jain]

Created attachment 402741 [details]
Patch

[Aakash Jain]

> Patch
Speculative fix.

[Aakash Jain]

Committed r263513: <https://trac.webkit.org/changeset/263513>

[Aakash Jain]

> Committed r263513: <https://trac.webkit.org/changeset/263513>
This change worked as expected, e.g.: https://bugs.webkit.org/show_bug.cgi?id=213602#c3

[Radar WebKit Bug Importer]

<rdar://problem/64781527>