Handle unparseable UniqueID consistently

[jlink]

Discovering tests via UniqueIdSelector and a string-based unique ID that cannot be parsed by UniqueId.parse() is handled differently by each engine. Should be handled consistently in the launcher.

I see several options:

• Throw (subtype of) JUnitException when executing the request or when creating the selector instance
• Ignore selector and log with warning

I'd prefer the first one since a syntactically wrong unique ID can only come from a bug by users of the launcher API. It's like violating a precondition.

[marcphilipp]

Original issue: #171.

[marcphilipp]

Additional Proposals:

• Return an AlwaysFailingTestDescriptor/DeadOnArrivalTestDescriptor/ErrorReportingTestDescriptor
• Pass reporting objec from Launcher into each TestEngine to report unparsable unique IDs, etc.