storage_mon: make -h exit just after printing help message (#1791)

• Make storage_mon -h exit just after printing help messages.

Previously, when -h or an invalid option was specified, storage_mon
printed the help messages, proceeded processing and then could
throw an error. This was not the behavior that, e.g., users who want
to specify -h option to see the help messages are expecting. To fix
this issue, this commit changes storage_mon so that it exits just
after printing the help messages when -h or an invalid option is
specified.

• Fix typo in help message.