Ensure that skipChildren() of non-blocking JsonParser will throw exception if not enough input

[cowtowncoder]

(note: see #462 as background)

Since non-blocking parser may not have enough content to successfully skip enough content to support skipChildren(), nor has any means to preserve or return enough state to allow caller to retry after content, it should indicate its inability to skip by throwing a JsonParseException.
Although it would be possible to throw this (or UnsupportedOperationException) for all calls, it may make more sense to only do this in cases operation can not succeed; and make it work if it can.