Fixes #3932: Prevent extent retrieval on the send datatype on non-root processes for MPI_Iscatter

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r29736 [[BR]]
Fix an error message for the igather and the usage of the extent on
non non-root processes for the iscatter. Thanks to Pierre
Jolivet for the bug report and the patch.

r29761 [[BR]]
Restrict the usage of MPI_Type_extent only to receiving processes
(aka the root). This commit is based on a patch provided by Pierre 
Jolivet.
Fix all the output to match the failing MPI call.