Move error_message JSON into own function

Right now all error messages requires a response object,
so they cannot be used directly in the json body. This
pulls the messages out into their own function so that
they can be used at any place in the response body.

Tested: It compiles and runs, verified return codes look
ok

Change-Id: I500b2164b2366086d610f0fdc8fa22985183e438
Signed-off-by: James Feist <james.feist@linux.intel.com>