Add JacksonException as parent class of JsonProcessingException (for 3.x compatibility)

[cowtowncoder]

Plan for Jackson 3.x includes big changes to exception handling (see https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-4), but one thing that might help future-proofing slightly would be to introduce the ultimate super type now in 2.12.
While JacksonException would still extend java.io.IOException (and this be checked), it could allow some changes to be made earlier, esp. for documentation.