Improve handling of non-reportable groovy classes

Due to Groovy AST bugs related with missing or corrupted information about source region (start row/column - end row/column) occupied by the given AST node, Clover has some heuristics implemented in order to skip such code sections.

However, this leads to the issue that some class may be omitted in the report without a warning.

Improvement:

• better heuristics for code regions related with classes
• print warnings to console about non-instrumented classes