Triage bug-mining errors

[rjust]

The initialize-revisions and analyze-project steps are iterative and can produce many warnings or errors, in particular in the first iteration. For example, an unknown project layout error usually applies to many bugs in the same project. Likewise, classpath issues are widespread.

Since many errors have the same root cause, it would be helpful if the bug-mining framework would produce a summary at the end, indicating which root cause(s) a user should address first before re-running the corresponding analysis.