More verbose error message when the request pools run out of space

[JiakunYan]

I am doing a message rate benchmark on MPICH with a super large send window (500k) using various numbers of VCIs. I found when I used more communicators than VCIs, I would get the following error:

Assertion failed in file ./src/include/mpir_request.h at line 450: req != NULL

After some (painful) debugging, I finally figured out it was due to the request pools running out of space.

It would be great if MPICH could make the error message more verbose than a plain req != NULL.