Errors handling in web services

Template:stubs I'm looking at how we return errors in the current Moodle web services. Here is the status:

- exceptions are raised: 16 functions

- string error messages: 2 functions (create notes / send messages)

- download/upload script return string error message (JSON) + error code

- When exception are raised, a unique error message is returned. It is format by the protocol server.

REST:

<error>

</error>