Improve ambiguous method error description

[sjaakd]

When doing name based automatically mapping it seems that mapstruct does not a clear hint what mapping goes wrong by stating a reference to the problematic mapping.

It could be that there's somewhere a List involved which breaks our current hierarchy.

Ambiguous mapping methods found for mapping property "org.mapstruct.ap.test.erroneous.ambigiousmapping.ErroneousMapper.LeafDTO leaf" to org.mapstruct.ap.test.erroneous.ambigiousmapping.ErroneousMapper.LeafEntity: org.mapstruct.ap.test.erroneous.ambigiousmapping.ErroneousMapper.LeafEntity map1(org.mapstruct.ap.test.erroneous.ambigiousmapping.ErroneousMapper.LeafDTO dto), org.mapstruct.ap.test.erroneous.ambigiousmapping.ErroneousMapper.LeafEntity map2(org.mapstruct.ap.test.erroneous.ambigiousmapping.ErroneousMapper.LeafDTO dto

does not offer a lot of info at one hand and is verbose in another way. If here's only one such branch leading up to a leaf, it's still to manage.. But if there are many you would like to have and indication which branch to look for..

So something like: Ambiguous mapping method found for source: trunk.branch.leaf with a short version of the types.

See also #2157