UncaughtThrowError is not defined on older Rubies

Author: pitr-ch

spec/concurrent/edge/erlang_actor_spec.rb

@@ -545,7 +545,7 @@
               expect(trapped_exit).to be_a Concurrent::ErlangActor::Exit
               expect(trapped_exit.from).to eq b
               expect(trapped_exit.reason).to eq b.terminated.reason
-              expect(trapped_exit.reason).to be_a UncaughtThrowError
+              expect(trapped_exit.reason).to be_a ArgumentError
               expect(trapped_exit.reason.message).to eq 'uncaught throw :uncaught'
             end
           end