Add NFSv4.1 and NFSv4.2 errors to nfsv4_errstr.h.

nfsv4_errstr.h only had strings for NFSv4.0 errors. This patch adds the
errors for NFSv4.1 and NFSv4.2. At this time, this file is not used by
any sources in the tree, so the change is not significant.
I do plan on using nfsv4_errstr.h in a future patch to mount_nfs.c.
Since I am doing this patch so that "minor version mismatch" will be
recognized, I made that string less abbreviated.