[chore] Tidy up some of the search logic

[tsmethurst]

GetRemoteAccount and GetRemoteStatus are now renamed in order to signal that you can pass a request for a local status/account into them, they'll just return the status/account early + not do any "remote" lookups. (I think in future instead of having these functions attached to a federator, we can have them attached to some kind of account/status 'store' which can explicitly return either local or remote things, as appropriate.)

Now that faux-remote lookups are properly guarded against in those functions, the processor search logic has been tidied up to just call those functions directly, rather than doing db calls first (and then pointlessly repeating them again in some cases).

Rather than allowing all errors coming from those functions to go ignored, this PR also introduces a new dereferencing.Error type and different versions thereof (see internal/federation/dereferencing/error.go). These types can be checked on by callers so that they only log errors where appropriate. This is now implemented in the processor: errors aren't logged by searching when something is just not retrievable, but if there's a database or significant transport error, they will be logged and result in an internal server error to denote that something is definitely not right.

Right now, this better error checking + returning is just implemented for the GetStatus and GetAccount functions, because I ran out of spoons. In future, we should also implement the error checking for dereferencing media and instances and whatnot.

This should help us debug #929 but it remains to be seen if it closes it.

[NyaaaWhatsUpDoc]

@tsmethurst let me know once this is passing tests / ready and i'll take a look :)

[tsmethurst]

@NyaaaWhatsUpDoc this is ready to go :)