error code consistency for memory objects

[bashbaug]

This is another PR to partially address the way we document error conditions, see #1320 and in particular #1320 (comment).

It contains the remainder of the changes from #1399, so when this PR is merged, #1399 can probably be closed as well.

[Kerilk]

Looks good to me, a clear improvement. In addition to the CL_EXEC_STATUS_ERROR_FOR_EVENTS_IN_WAIT_LIST discrepancy, I'd note that some call use CL_INVALID_CONTEXT for when events and command queue disagree on context, others (like clEnqueueReleaseExternalMemObjectsKHR) use CL_INVALID_COMMAND_QUEUE, and others like clEnqueueAcquireExternalMemObjectsKHR use neither, so I assume would default to CL_INVALID_EVENT_WAIT_LIST.