libzfs: Rename msg bufs to errbuf for consistency

libzfs_pool.c uses the name msg where everywhere else in libzfs uses
errbuf for the error message buffer.

Use the name consistent with the rest of libzfs and use ERRBUFLEN
instead of 1024.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #13539