bectl: make mount subcommand less verbose

[rew]

Summary

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

[asomers]

Works for me.

[kevans]

I'm happy