Start testing errors.

- Add support to test runner for compile errors.
- Include line number in tokens.
- Exit with non-zero on compile error.

test/undefined_global.wren

@@ -0,0 +1 @@
+io.write(notDefined)  // expect error

test/undefined_local.wren

@@ -0,0 +1,3 @@
+{
+  io.write(notDefined)  // expect error
+}