Better handling when growlnotify is not installed

Better handling when growlnotify is not installed

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: Wincent Colaiuta <win@wincent.com>