Handle JsonMappingExceptions (issue #21)

Addresses #21 by adding a case for [`JsonMappingException`](http://fasterxml.github.io/jackson-databind/javadoc/2.4/com/fasterxml/jackson/databind/JsonMappingException.html) to `HttpResourceActor.handleError`.
merge = put the two `case` clauses together. their bodies are doing the exact same thing, so unless they're different we should merge them