Errata: fix a couple of spots in server upcall functions

[rhc54]

Server upcall functions were required to return prior to executing the callback function. This was a limitation of the implementation and not a requirement of the Standard - the statements should never have been included here.

FWIW: the limitation has subsequently been removed from the reference implementation.

In the pmix_server_iof_fn_t, clarify that an error status can be returned either directly by the function or in the callback function. It does not require that the host execute the callback to return an error, as was implied by the current language.

[rhc54]

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