Bug overview description.
I frequently add posts to pools with pool:xxxxxx tag, not via pool editor GUI. I've noticed that when you try to add a post to a pool that doesn't exist yet, it always produces a crash (he's dead, Jim). Well, I guess this doesn't happen all that often, but IMO it wouldn't hurt to add a check and some error message for this situation.

What part(s) of the site page(s) are affected?
Pools.

What is the expected behavior?
No crash, possibly an error message or other user-friendly resolution (discussed below).

What actual behavior is given instead?
Crash (he's dead, Jim).

Time of incident (if applicable).
Happens always.

Can you reproduce the bug every time?
Yes.

What steps did you take to replicate this bug?
Just try to add pool:200000 or something like that to any post.

Errors or other messages returned (if any).
He's dead, Jim.

Once I was unable to upload a post and I couldn't figure out why and it turns out I had an invalid pool:##### tag. It would be nice if invalid tags of that sort simply vanished instead of preventing the upload altogether.

IMO the error message option would be better than vanishing, because that way uploader knows what are they doing wrong. Alternatively, non-existing pool tags could be aliased to invalid_tag.