Generate a runtime error if a foreign method was not found.

test/language/foreign/foreign_after_static.wren

@@ -0,0 +1,3 @@
+class Foo {
+  static foreign method // expect error
+}

test/language/foreign/foreign_method_with_body.wren

@@ -0,0 +1,3 @@
+class Foo {
+  foreign method { "body" } // expect error
+}

test/language/foreign/unknown_method.wren

@@ -0,0 +1,3 @@
+class Foo {
+  foreign someUnknownMethod // expect runtime error: Could not find foreign method 'someUnknownMethod' for class Foo in module 'main'.
+}