Is your feature request related to a problem? Please describe.
OS_ERR_INVALID_PRIORITY error code is defined but not used anywhere. Documented as a return code for both OS_TaskCreate and OS_TaskSetPriority APIs.

Describe the solution you'd like
Use or remove.

Describe alternatives you've considered
Could leave in if it's just not applicable to any of the open source OS's, but seems strange it wouldn't apply for at least one of them.

Additional context
Discovered in unit/coverage testing. Relation to #222.

Requester Info
Jacob Hageman - NASA/GSFC