In the case of search results from a given mirror server (that uses search.musicbrainz.org) being throttled, it looks like there's no special error, meaning the error returned is:

Sorry, but your query (HASH(0x1153ebe0)) could not be performed, due to an error which we are not quite able to identify.

Which is really, really unuseful for determining what the heck's going on.

We should change that.