py: Implement UnicodeError.

Still too shy to implement UnicodeEncodeError which was really needed for
micropython-lib case.
diff --git a/py/objexcept.c b/py/objexcept.c
index 858804d..824c9b0 100644
--- a/py/objexcept.c
+++ b/py/objexcept.c
@@ -249,7 +249,11 @@
   //MP_DEFINE_EXCEPTION(SystemError, Exception)
   MP_DEFINE_EXCEPTION(TypeError, Exception)
   MP_DEFINE_EXCEPTION(ValueError, Exception)
-    //TODO: Implement UnicodeErrors which take arguments
+#if MICROPY_PY_BUILTINS_STR_UNICODE
+    MP_DEFINE_EXCEPTION_BASE(ValueError)
+    MP_DEFINE_EXCEPTION(UnicodeError, ValueError)
+    //TODO: Implement more UnicodeError subclasses which take arguments
+#endif
   /*
   MP_DEFINE_EXCEPTION(Warning, Exception)
     MP_DEFINE_EXCEPTION_BASE(Warning)