Changed `extern crate foo as bar;` error message

I did not put the crate name in the error note, if that's necessary I'll look into it.

Closes #17709
Yes, it did hit two messages, but it didn't seem horrible to have both a note and an error (but it's my first diagnostic issue, so it may be why ^^).
On it, I'll commit you a new one in about an hour :)