py: Make viper codegen raise proper exception (ViperTypeError) on error.

This fixes a long standing problem that viper code generation gave
terrible error messages, and actually no errors on pyboard where
assertions are disabled.

Now all compile-time errors are raised as proper Python exceptions, and
are of type ViperTypeError.

Addresses issue #940.

py/objexcept.c

@@ -252,6 +252,10 @@ MP_DEFINE_EXCEPTION(Exception, BaseException)
       */
   //MP_DEFINE_EXCEPTION(SystemError, Exception)
   MP_DEFINE_EXCEPTION(TypeError, Exception)
+#if MICROPY_EMIT_NATIVE
+    MP_DEFINE_EXCEPTION_BASE(TypeError)
+    MP_DEFINE_EXCEPTION(ViperTypeError, TypeError)
+#endif
   MP_DEFINE_EXCEPTION(ValueError, Exception)
 #if MICROPY_PY_BUILTINS_STR_UNICODE
     MP_DEFINE_EXCEPTION_BASE(ValueError)