loader: provide error message in command_errmsg instead of printing
Also provide an error message when argc is too long instead of printing
"no error message".
Reviewed by: imp, kib
Pull Request: https://github.com/freebsd/freebsd-src/pull/1381
(cherry picked from commit 6fdb07d5b7e9293a6851a23b7ecb26636cf1ec1e)