Add handle_caps_mismatch(?) callback

[mat-hek]

Originally posted in #327 by @bblaszkow06

In order to allow providing users with meaningful feedback when there's a caps mismatch on a pad, we could add a callback like handle_caps_mismatch or handle_invalid_caps to WithInputPads behaviour allowing to print custom messages.

The examples where this could be useful:

• MP4 Muxer that requires payloaded media streams and could suggest adding them
• Elements requiring parsed input like H264 encoder and decoder