improve error reporting when a required parameter is missing on mvc routes

[jknack]

@Path("/issue177")
  public static class Resource {

    @GET
    public String param(final String x) {
      return x;
    }

  }

A request like: GET /issue177 result in a 400 response with a message like: No parser for java.lang.String

Error response need to be more clear/specific, like: Bad Request(400): Not found: x