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 be printed to stderr and bectl will
exit with a non-zero value.
PR: 273180