Changed phrasing, fixed and added a test.

test/method/not_found.wren

@@ -1,3 +1,3 @@
 class Foo {}
 
-(new Foo).someUnknownMethod // expect runtime error: Receiver does not implement method 'someUnknownMethod'.
+(new Foo).someUnknownMethod // expect runtime error: Foo does not implement method 'someUnknownMethod'.

test/method/static_method_not_found.wren

@@ -0,0 +1,3 @@
+class Foo {}
+
+Foo.bar // expect runtime error: Foo metaclass does not implement method 'bar'.