Part of MDL-66074

This change modifies the return val for all of the grading functions to
allow them to add additional information.
This means that a grading service can suppress a Grade saved message if
there were no changes, for example.

This also adds a distinction between:
- Errored (Exception thrown in the WS call)
- Failed (Warning in the output of the WS call)
- Success (Grade actually saved)
- None of the above (No save, no fail, no change)