Throwing exception when checker fails to run

[mccartney]

Scalastyle checker silently swallows exceptions, e.g. Scalastyle runs fine with the following configuration file reporting no errors:

<scalastyle>
  <name>Scalastyle configuration</name>
  <check level="error" class="this.does.not.Exist!" enabled="true"/>
</scalastyle>

It's error-prone. A project can keep thinking its configuration is right, while it's a simple typo in class name.

The downside of the solution is that it reports an error for each checked file. I believe it's strictly better than the current swallowing though.

[codecov-io]

Codecov Report

Merging #297 into master will not change coverage.
The diff coverage is 0%.

@@          Coverage Diff          @@
##           master   #297   +/-   ##
=====================================
  Coverage       0%     0%           
=====================================
  Files          59     59           
  Lines        1464   1464           
  Branches      147    147           
=====================================
  Misses       1464   1464

Impacted Files	Coverage Δ
src/main/scala/org/scalastyle/Checker.scala	0% <0%> (ø)	⬆️

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[matthewfarwell]

Thanks for thie PR. I need to think about this, and I'll get back to you.