[debugger] Run Download callback earlier

Previously, the Download callback wouldn't run until all pending
requests had finished. This means all the servers we initially query
must at least respond to the initial check request before we get the
results of our download. With this patch we run the callback as soon as
we have the symbols down. This should also prevent a potential race
where server checks that fail after we've already gotten the symbols
could change our error status.

Change-Id: I7db38e1b5593921594792d39e42164cd0184b4c6