When encountering EZFS_UNKNOWN, print the error text buffer anyway

Rather than just saying there was an internal error, provide any
context we might have to the user to help them understand the issue.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Closes #10632