Include parser context in error msgs.

Currently, eitherDecodeWith drops the attoparsec context when
returning a Left value.  In box/attoparsec#79, the context is
included in the output of L.eitherResult.  Since L.parseOnly
does not exist, the closest we can get to the structure of
eitherDecodeStrictWith here in aeson is L.eitherResult . L.parse.