search: use a queue to collect errors

The threading in the tap search code makes handling errors difficult. If
an API-related error is raised in one thread, it is likely to be raised
in each of the rest as well. This results in duplicated error messages,
which is ugly and bad UX.

This patch adds a synchronized queue to collect these exceptions. The
first one added to the queue is re-raised after all operations are
complete.

It's not ideal, but it's minimally invasive and I don't have the energy
or time to do a rewrite.