Fix error resposes for `from` search prefix (#17963)

* Fix error responses in `from` search prefix (addresses mastodon/mastodon#17941)

Using unsupported prefixes now reports a 422; searching for posts from an
account the instance is not aware of reports a 404. TODO: The UI for this
on the front end is abysmal.

Searching `from:username@domain` now succeeds when `domain` is the local
domain; searching `from:@username(@domain)?` now works as expected.

* Remove unused methods on new Error classes as they are not being used

Currently when `raise`d there are error messages being supplied, but
this is not actually being used. The associated `raise`s have been
edited accordingly.

* Remove needless comments

* Satisfy rubocop

* Try fixing tests being unable to find AccountFindingConcern methods

* Satisfy rubocop

* Simplify `from` prefix logic

This incorporates @ClearlyClaire's suggestion (see
https://github.com/mastodon/mastodon/pull/17963#pullrequestreview-933986737).

Accepctable account strings in `from:` clauses are more lenient than
before this commit; for example, `from:@user@example.org@asnteo +cat`
will not error, and return posts by @user@example.org containing the
word "cat". This is more consistent with how Mastodon matches mentions
in statuses. In addition, `from` clauses will not be checked for
syntatically invalid usernames or domain names, simply 404ing when
`Account.find_remote!` raises ActiveRecord::NotFound.

New code for this PR that is no longer used has been removed.

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