Check rc and print error message if any.

It is probably good to use stdout before printing a generic error
message as well.
Fail if "rcctl status" writes to stderr.
Tweak error checking for "enabled" code.

Based on input from @jarmani:
* A return value of 2 now means a service does not exist. Instead of
  trying to handle the different meanings of rc after running "status",
  just look at stderr to know if something failed.
* Skip looking at stdout to make the code cleaner. Any errors should
  turn up on stderr.
Merge pull request #2 from eest/jarmani-openbsd_rcctl

Check rc and print error message if any.