Fix arcstat.py handling of unsupported options

This change allows the arcstat.py script to handle unsupported options
gracefully and print both error and usage messages when one such option
is provided.

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7799

cmd/arcstat/arcstat.py

@@ -285,7 +285,7 @@ def init():
             ]
         )
     except getopt.error as msg:
-        sys.stderr.write(msg)
+        sys.stderr.write("Error: %s\n" % str(msg))
         usage()
         opts = None