Better handling when growlnotify is not installed (WOTest, f0adf26)

Previouly when growlnotify was not installed env would issue an ugly error message to the standard error.

This commit redirects standard error to /dev/null so as to suppress this output and instead checks the exit status of the env tool. When growlnotify cannot be found in the current PATH a note (not an error or warning) is printed to the console. Other failure cases issue either an error or a warning, depending on their severity.

Signed-off-by: Greg Hurrell <greg@hurrell.net>