Change broctl check command output to be more specific

Changed the output of the check command to be more specific about
what it is actually checking (the existing message "worker-1 is ok"
could be misleading if we cannot even connect to the worker-1 host).
Fix a failing test on FreeBSD 10
Better error handling for "check" and "scripts" commands

Improved the "check" and "scripts" commands to report a single error
message (instead of multiple confusing messages) when a user forgets to
first do an "install".  Updated documentation of "check" command to
remove a statement about check being the only command that works without
first doing an install.

Also fixed exit status to be non-zero when the check or scripts commands
fail to create or remove tmp directory.
Better error handling for broctl cron command

Added an error message if someone tries to run broctl cron without
first doing an install (in order to avoid running scripts with undefined
variables which could have bad side effects).
Better error handling for broctl df command

Added output of error message when df helper fails.  Added error check
to df helper for when broctl install needs to be run.
Better error handling for broctl process command

Improve error output when broctl install has not been run yet.