Add StreamWriteException type to eventually replace JsonGenerationException

[cowtowncoder]

(another issue related to #661 -- but this one can go in 2.13)

In 2.10 a new intermediate exception type StreamReadException was added, to help change exception hierarchy in 3.0 to have less JSON-specific names (and some other related cleanup). It would make sense to similarly add StreamWriteException and this can go in 2.13 even if the main goal is to improve 3.0: this may help make transition from 2.x to 3.0 slightly easier. Exception should extends JsonProcessingException in 2.x; may rethink this for 3.0 (to directly extend JacksonException, likely).