Unify missing app error messages
Add proper error handling to each config command
Simplify env var checking in config plugin

All checks are now done within the commands, and we only need to ensure the config file exists.
Simplify if statement
Correct plugin example

$APP is not available until after the if guards
Unify the way in which all error messages are specified

Other parts of dokku use one-liners where possible, so these parts should as well
Exit 0 if there is no configuration

heroku config:get exits 0 in this case
Merge pull request #782 from progrium/simplified-config

Simplified config