Ensure JPlag fails gracefully if JavaC is not available and Java code is analyzed

[tsaglam]

Sometimes users run JPlag without a full JDK, and thus JavaC is not available. This manifests in an error like this:

Exception in thread "main" java.lang.NoClassDefFoundError: com/sun/source/util/SourcePositions
        at de.jplag.java.Parser.parse(Parser.java:24)

To this end, this PR checks if JavaC is available by trying to load a corresponding class. If this does not work, it fails gracefully. To implement this, the exceptions of the API were extended: Language modules can now throw a CriticalParsingException, which leads to termination. The core uses a new subclass of ExitException to wrap this and pass it to the top.

[sonarqubecloud]

Quality Gate failed for 'JPlag Plagiarism Detector'

Failed conditions
46.7% Coverage on New Code (required ≥ 50%)

See analysis details on SonarCloud