bectl: make mount subcommand less verbose
The mount subcommand currently produces output such as:
- bectl mount <bootenv> Successfully mounted <bootenv> at <mountpoint>
This commit changes it to only print the mountpoint:
- bectl mount <bootenv> <mountpoint>
This makes it easier to script the mount subcommand. If an error occurs
while mounting, an error message is printed to stderr and bectl will
exit with a non-zero value.
PR: 273180
Reviewed by: kevans, asomers
Differential Revision: https://reviews.freebsd.org/D41562