JSTEP-4: Changing `JsonProcessingException` to be unchecked

[cowtowncoder]

This is the placeholder issue for JSTEP that defines exact details of how Jackson 3.0 will change exceptions used, so that what is now JsonProcessingException (checked exception) will be converted to a new unchecked exception type, most likely with new name (JacksonException). It will also define how wrapping works wrt. IOExceptions that were formerly passed as-is.

Some existing thoughts:

• https://github.com/FasterXML/jackson-databind/issues/2177 (Change parent type of JsonProcessingException to be RuntimeException)
• JsonMappingException -> DatabindException (and other renaming)
• How to unify location handling between "streaming"/"databind" (former JsonProcessingException vs JsonMappingException)

[cowtowncoder]

Now that https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-4 is more fleshed out, closing placeholder.