add info about ErrorWrapper object

What problem does this feature solve?

As I can see find method returns Wrapper object if element found or ErrorWrapper object if not element found. Both are implement BaseWrapper.

I think it will be good to add this information in order to do more clear info about returning type of find.

Also, may be create page for ErrorWrapper object.

What does the proposed API look like?

Returns: {Wrapper}

to something like

Returns: {Wrapper | ErrorWrapper}

...some notes when ErrorWrapper returned and example...

[Robin-Hoodie]

Testing this with @vue/test-utils@1.0.3 and exists() actually returns false for an ErrorWrapper component

This can also be found in the sourcecode: https://github.com/vuejs/vue-test-utils/blob/4b0c5c94fb29885bb65e26fea64b8ca64960e301/packages/test-utils/src/error-wrapper.js#L74

[tomasbjerre]

I prefer naming methods like:

• find(x) that will search for x. It may not find anything, and it is totally expected.
• get(x) that will get x. It will always return value of x or it will throw exception stating that x could not be found.

So I am missing the get method here.

See #1298