Mug#sip raises error if Mug is empty

* An empty mug can't be drinked from unless you like drinking _AIR_!
:sparkles:

[#1337]

mug.rb

@@ -20,6 +20,7 @@ def empty?
   end
 
   def sip
+    raise "You're all out of #{content}" if empty?
     @percent_empty += 1
   end
 end