Improve non-interactive forth cmd error reporting

Non-interactive forth command errors where silent even for critical issues
e.g. failing to load a required kernel module or mfs_root.

This resulted in later unexplained and hard to trace errors such as mount
root failures.

This introduces additional command return codes that are treated
appropriately by the non-interactive command processor (bf_command).

• CMD_CRIT = print error
• CMD_FATAL = panic

Also fix minor style(9) issues with command_load return codes.

MFC after: 2 weeks
X-MFC-With: r293268
Sponsored by: Multiplay