Add context to error message if encryption key is missing

I was deploying a fresh install to Heroku and this tripped me up for 15 minutes.

Author: Gary Moore

lib/encryption.rb

@@ -22,7 +22,7 @@ def self.decrypt_sensitive_value(val = "")
 
   # Should be able to just re-use the same key we already have!
   def self.key
-    raise "Key Missing" if !(KEY)
+    raise "Key Missing. Add one in initializers/key.rb" if !(KEY)
     KEY
   end