Add InvalidTransition exception with more helpful message.

lib/stateful_enum/invalid_transition.rb

@@ -0,0 +1,11 @@
+module StatefulEnum
+  class InvalidTransition < StandardError
+    attr_reader :state
+    attr_reader :event
+
+    def initialize(state, event)
+      @state, @event = state, event
+      super("Invalid transition from state #{@state.inspect} via event #{@event.inspect}")
+    end
+  end
+end

lib/stateful_enum/machine.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'stateful_enum/invalid_transition'
+
 module StatefulEnum
   class Machine
     attr_reader :events
@@ -66,9 +68,10 @@ def initialize(model, column, states, prefix, suffix, name, &block)
           end
 
           # def assign!()
-          detect_enum_conflict! column, "#{value_method_name}!"
-          define_method "#{value_method_name}!" do
-            send(value_method_name) || raise('Invalid transition')
+          value_method_name_bang = "#{value_method_name}!"
+          detect_enum_conflict! column, value_method_name_bang
+          define_method value_method_name_bang do
+            send(value_method_name) || raise(InvalidTransition.new(send(column), value_method_name_bang.to_sym))
           end
 
           # def can_assign?()

test/mechanic_machine_test.rb

@@ -45,10 +45,14 @@ def test_invalid_transition
   def test_invalid_transition!
     bug = Bug.new
     bug.resolve!
-    assert_raises do
-      bug.assign!
-    end
+    exception =
+      assert_raises(StatefulEnum::InvalidTransition) do
+        bug.assign!
+      end
     assert_equal 'resolved', bug.status
+    assert_equal 'Invalid transition from state "resolved" via event :assign!', exception.message
+    assert_equal 'resolved', exception.state
+    assert_equal :assign!, exception.event
   end
 
   def test_can_xxxx?