Output a friendly message when no type given for GeneratedAttribute [#5461 state:resolved]

Signed-off-by: José Valim <jose.valim@gmail.com>

railties/lib/rails/generators/generated_attribute.rb

@@ -6,6 +6,7 @@ module Rails
       attr_accessor :name, :type
 
       def initialize(name, type)
+        raise Thor::Error, "Missing type for attribute '#{name}'.\nExample: '#{name}:string' where string is the type." if type.blank?
         @name, @type = name, type.to_sym
       end