ncmpi_sterror: slight behavior change.

[Rob Latham]

Working towards a release, I promise.

ncmpi_strerror /nfmpi_strerror turns a parallel-netcdf error code into
a human readable string.  NC_EBADID becomes "NetCDF: Not a valid ID",
for example.  

If someone passes in a bogus error code, serial netcdf returns the
string "Unknown Error". 

We used to do that but decided it would be better to have a more
helpful error message:

"Unrecognized pnetcdf error code: ID" (where ID is the unrecognized
error id).

I just wanted to double check with the wider pnetcdf community that
such a change in behavior would be OK.  It's sort of a programming
interface change, but I hope no one (or more accurately, no computer
program or testing script) was looking for a literal "Unknown Error"
from ncmpi_error.

==rob

-- 
Rob Latham
Mathematics and Computer Science Division
Argonne National Lab, IL USA