Allow messages as well as error codes in response to EXEC messages

[amcewen]

The wire protocol allows for a message to be returned along with a failure message when you invoke a specific step. The wire-server treats any non-0 response from an EXEC call to be a fail, and includes an error message but it's just a generic "Failed with $error_code".

It will be useful for more descriptive errors to be returned, and they would (/could - maybe there could also be an error lookup in the wire-server?) be best defined in the firmware running on the microcontroller.

The wire-server should expect an error-code initially, and then the remainder of the line should be assumed to be a message to return alongside the failure response.