Give a clearer error message when payload doesn't match with provided content-type

Let we are not support application/json content-type.
When user send a request with wrong content-type, the response message is clear:

{
  "error" => "The provided content-type 'application/json' is not supported"
}

Let we support application/json content-type.
When user send a request with correct content-type, but with wrong payload (the format of it is not json),
the response message will be strange:

{
  "error" => "Empty input (after ) at line 1, column 1 [parse.c:1072] in '......"
}

It will be nice if we can provide clearer error message.
But it seems to be impossible to add custom messages without overriding codes in the gem.

The error is throwed and rescued inside:

https://github.com/ruby-grape/grape/blob/f5d9831bac2e2dd439d0f3901797995a91139690/lib/grape/middleware/formatter.rb#L115-L116
https://github.com/ruby-grape/grape/blob/f5d9831bac2e2dd439d0f3901797995a91139690/lib/grape/middleware/error.rb#L38-L40

Start in lib/grape/middleware/formatter.rb around lines 115-116 and lib/grape/middleware/error.rb around lines 38-40, then reproduce a request using application/json with an invalid JSON payload. Trace the exception through formatting and rescue handling to determine where a clearer response can be introduced. Done means malformed JSON produces a clear error message instead of the parser's raw message.