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/modbuiltins.c

@@ -703,6 +703,9 @@ STATIC const mp_map_elem_t mp_module_builtins_globals_table[] = {
     { MP_OBJ_NEW_QSTR(MP_QSTR_UnicodeError), (mp_obj_t)&mp_type_UnicodeError },
     #endif
     { MP_OBJ_NEW_QSTR(MP_QSTR_ValueError), (mp_obj_t)&mp_type_ValueError },
+    #if MICROPY_EMIT_NATIVE
+    { MP_OBJ_NEW_QSTR(MP_QSTR_ViperTypeError), (mp_obj_t)&mp_type_ViperTypeError },
+    #endif
     { MP_OBJ_NEW_QSTR(MP_QSTR_ZeroDivisionError), (mp_obj_t)&mp_type_ZeroDivisionError },
     // Somehow CPython managed to have OverflowError not inherit from ValueError ;-/
     // TODO: For MICROPY_CPYTHON_COMPAT==0 use ValueError to avoid exc proliferation