exceptions: add FormulaMethodDeprecatedError.

This will be raised when Homebrew developers try to use formulae that
call deprecated methods.

Library/Homebrew/exceptions.rb

@@ -56,6 +56,8 @@ end
 
 class FormulaSpecificationError < StandardError; end
 
+class FormulaMethodDeprecatedError < StandardError; end
+
 class FormulaUnavailableError < RuntimeError
   attr_reader :name
   attr_accessor :dependent