Fix #1103, Implement OS_ERR_INVALID_PRIORITY return code check for RTEMS

[thnkslprpt]

Checklist

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to GSFC-SoftwareRelease@mail.nasa.gov and copied cfs-program@lists.nasa.gov.

Describe the contribution

• Fixes OS_ERR_INVALID_PRIORITY not used #1103
  • Implements the OS_ERR_INVALID_PRIORITY error return code for RTEMS (in OS_TaskCreate()) and OS_TaskSetPriority().

For POSIX, OSAL uses the internal function OS_PriorityRemap() to convert to a suitable (and not invalid) priority value before calling pthread_setschedprio().

For VxWorks, taskPrioritySet() does set ERRNO to S_taskLib_ILLEGAL_PRIORITY, but I have not tried to include it in this PR. The issue can be left open if someone wants to look at that, or closed for now with this PR.

Testing performed
I don't think this file (rtems/src/os-impl-tasks.c) has any tests currently (see #1207).
If someone can advise if this needs to be tested, and the best way of doing so, I would appreciate it.

Expected behavior changes
Calls on the RTEMS implementation to OS_TaskCreate() (which calls rtems_task_create()) and OS_TaskSetPriority() (which calls rtems_task_set_priority()) will now actually return OS_ERR_INVALID_PRIORITY if the RTEMS calls return RTEMS_INVALID_PRIORITY, rather than simply returning OS_ERROR.

Contributor Info
Avi Weiss @thnkslprpt

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