Introduce custom exception for missing APP_KEY

[bambamboole]

Hey guys,

I am building an application with an interactive installer.
My first thought was to catch the missing APP_KEY exception to trigger the installation process.

After a short research I found that there is only a RuntimeException thrown.

Now my question:

Would You accept a pull request which introduces a custom MissingAppKeyException for the sake of a better catch option?

Keeps safe