Improve unimplemented message

[lily-mara]

When calling Bridged::bridge, it is possible to get a confusing
unimplemented error. This PR adds some explanation and clarity around
the error.

[hgzimmerman]

I like the idea of this, but am hesitant about linking to a specific version of the documentation, as that is likely to fall out of sync quickly when we bump the version.
I believe that https://docs.rs/yew/latest/yew/agent/index.html will redirect to the newest documentation version, so barring significant reorganization of the project, the link will continue to point at the most relevant documentation without any intervention on our part.

[lily-mara]

Does this message look better?

[hgzimmerman]

Yes, I think its satisfactory. I don't have merge privileges on this repo, so you need to wait for a maintainer with the ability to merge to take a look at this. Sorry for the delay.

[jstarry]

@natemara sorry for the delay, can you fix the fmt error that is showing up on the rust stable ci job?

It's pretty unfortunate that you ran into this, the Global agent is not implemented at all yet so I'm not sure why it was added in the first place. If you're interested in using it, here's an issue to tackle if you're up for the task! #283