adjust the text of dartdoc exit messages

[devoncarew]

adjust the text of dartdoc exit messages:

• avoid printing a stack trace for exceptions used to signal an exit code
• have the 'found xxx issues' message use sentence case
• remove a double period for the 'dartdoc failed' message
• avoid saying 'dartdoc' twice in the 'dartdoc failed' message

Before these changes:

found 50 warnings and 107 errors

dartdoc failed: dartdoc encountered 107 errors while processing..
#0      Dartdoc.generateDocs (package:dartdoc/dartdoc.dart:231:9)
<asynchronous suspension>
#1      Dartdoc.executeGuarded.<anonymous closure> (package:dartdoc/dartdoc.dart:500:9)
<asynchronous suspension>

and after:

Found 50 warnings and 107 errors.

dartdoc failed: encountered 107 errors.

[devoncarew]

I believe this failure:

https://github.com/dart-lang/dartdoc/pull/2489/checks#step:6:3572

dart2-model_special_cases_test.dart:     Actual: '<p>Text.</p>\n'
dart2-model_special_cases_test.dart:               '<p>Info: Compiling without sound null safety</p>\n'
dart2-model_special_cases_test.dart:               '<p><div class="title">Title</div></p>'

Is due to a (front end?) change which emits a new message to stdout. We should either suppress the message with a cli flag, or ask that the stdout message be reverted.

[devoncarew]

I think we'll need to pass --verbosity=warnings into some compile step.

[coveralls]

Coverage increased (+0.01%) to 91.616% when pulling 57d1446 on devoncarew:adjust_exit_messages into 0ba8dc8 on dart-lang:master.