Add themeRequestError reducer and getThemeRequestError selector and tests.

[budzanowski]

Info

The new themes Redux sub-tree requires error handling. This PR adds reducer and selector that that respond to theme request action success or error.

Testing

Not connected yet. Will be integrated in #9750,

[matticbot]

Test live: https://calypso.live/?branch=add/themeRequestsError-reducer-and-tests

[seear]

Looks good.

For our use case we could store a boolean instead of the actual error object, since I don't think we ever make use of it. But I don't see any harm in storing the error.

[seear]

Alternatively:
[ siteId ]: omit( state[ siteId ], [ themeId ] ),

[ockham]

Without the brackets around themeId, I think, tho...

[ockham]

The variable name is misleading here since we're getting an error, not a themes array.

I think the whole selector could be shortened to

return get( state.themes.themeRequestsError, [ siteId, themeId ], null );

thanks to the way get works. (You could consider also dropping the fallback null -- it will then just return undefined, which should be expressive enough too, I think.)

[ockham]

This will produce a themeRequestsError state like this (stolen from your unit tests 😛 ):

2916284: {
	vivaro: {
		error: 'Request error'
	}
 }

Let's drop the error layer since it bears no extra information:

2916284: {
	vivaro: 'Request error'
 }

i.e. just omit the curly braces around error so it doesn't produce an object:

[ siteId ]: Object.assign( {}, state[ siteId ], { [ themeId ]: error } )

which I think you can then write using destructuring as

[ siteId ]: {
	...state[ siteId ],
	[ themeId ]: error
}

[ockham]

Naming nitpick: themeRequestErrors (since our other reducers are plural, too; the Request middle part should be singular, tho, I think 😉 ).

[ockham]

Left a couple of notes. Good to merge after you've addressed them. Love your unit tests, those are really helpful when reviewing and reasoning about state shape.

[ockham]

👍 Good spot.