Add "current token" info into `MismatchedInputException`

[cowtowncoder]

(follow-up to FasterXML/jackson-core#1438)

Since "current token" (JsonParser.currentToken()) will be cleared when parser is closed, information from MismatchedInputException and sub-types won't be available if exception handling calls close().
This occurs typically with higher-level calls, like jackson-databind "readValue()" calls via ObjectMapper and ObjectReader, where JsonParser is auto-closed.

As one thing to address this, let's get and store _currToken as an accessible property of MismatchedInputException.