Update error message in the Store

[dmitry-kurmanov]

Updated error message in the Store for the better understanding what is happenning.

And it improves code readability I think.

[dmitry-kurmanov]

@Rich-Harris or may be we need something like

computed property is read only

PS I think this is not a feature so I have not created an issue. Am I wrong?

[Rich-Harris]

I think the new message is good — thank you!