[2.7] Add back-references to `JsonParser` / `JsonGenerator` for low-level parsing issues

[cowtowncoder]

When JSON encoding/decoding fails, it would make sense to have access to the underlying parser/generator, from catch block. Although instance may not be in mint condition (depending on how error handling within Jackson works), it might still give useful low-level information on actual failure, above and beyond what location information Exception itself contains.
For example: JsonParser.getCurrentName() may be available, as we all JsonParser.getParsingContext().

[cowtowncoder]

Implemented for 2.7.0.