non negative error codes to PVFS_perror_gossip()

[carns]

I have an old note about getting a segfault out of PVFS_perror_gossip(), but I have lost the context of what caused it. I think the problem may have been that a positive value error code was accidentally passed in.

Need to confirm if this is a problem and make it so that this case does not segfault. It would be better if it just printed an "invalid error code" message and showed the decimal value.