Delete/exit/create child task additional error codes need to be documented in API and tested (related to requirements)

[skliper]

Is your feature request related to a problem? Please describe.
We've got requirements to error if CFE_ES_DeleteChildTask or CFE_ES_ExitChildTask relate to the main app task:

cFE/docs/cFE_FunctionalRequirements.csv

Line 114 in c4ae5b2

ES: Delete Child Task - Error If Application Main Task,cES1312.1,"If the specified task is the cFE Application Main Task, the request shall record the error in the System Log, and return an error code.",Cannot use Child Task Exit or Delete on the cFE Application Main Task.

cFE/docs/cFE_FunctionalRequirements.csv

Line 116 in c4ae5b2

ES: End Child Task - Error If Application Main Task,cES1314.1,"If the calling task is the cFE Application Main Task, the cFE shall record the error in the System Log, and return an error code.",Cannot use Child Task Exit or Delete on the cFE Application Main Task.

Also CFE_ES_CreateChildTask can not be called from a child task:

cFE/docs/cFE_FunctionalRequirements.csv

Line 112 in c4ae5b2

ES: Create Child Task - Invalid From Child Task,cES1311.2,"In the event a child task attempts to create another child task, the cFE shall record the error in the System Log, and return an error code.",Only the cFE Application's main task can create a child task. This prevents confusion with parent/child task relationships and the allocation/deallocation of resources.

Describe the solution you'd like
Document in the API and exercise these cases and test for return codes:
CFE_ES_ERR_CHILD_TASK_DELETE_MAIN_TASK
CFE_ES_ERR_CHILD_TASK_CREATE
Note CFE_ES_ExitChildTask is a void so no error code to check but shouldn't exit from a main app, exercise from main test app task to confirm

Describe alternatives you've considered
None

Additional context
Hopefully this would be caught by the upcoming API scrub (@jphickey), but documenting explicitly since it's a requirement verification issue. Not sure if it's easier to fix now or with the rest of the updates... open to whatever is easiest.

Requester Info
Jacob Hageman - NASA/GSFC

[jphickey]

Note - as part of previous PR #1869 this adds test cases that cover the following cases of CFE_ES_ERR_CHILD_TASK_DELETE_MAIN_TASK and CFE_ES_ERR_CHILD_TASK_CREATE status codes from the respective APIs (prevents creating child from child, or deleting main task from child, etc).

The remaining work item is to invoke CFE_ES_ExitChildTask