failsOnError should fail the build, not abort the report #656

[AzazelSensei]

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failsOnError used to throw CheckstyleExecutorException from DefaultCheckstyleExecutor. That is a plugin error, and it also stops the site report from rendering (the TODO on that throw).

The executor now records the message on CheckstyleResults. The check goal turns it into MojoFailureException. The report goal still generates the site.

CheckstyleViolationCheckMojoTest.testPlainOutputFileFailOnError now expects the failure, not an execution error.

Fixes #656