crash_dump: avoid misleading error messages.

I'm guessing that the original

  F crash_dump64: crash_dump.cpp:460] failed to attach to thread 1671, already traced by 0 ()

was probably a race, where there _was_ a tracer but they disappeared?
Whatever, it doesn't seem helpful to show "already traced by nobody",
and we also don't want to clobber errno in the fallthrough case
(previously just where get_tracer() failed, but now also where
get_tracer() returns "nobody").

Bug: http://b/188668580
Test: treehugger
Change-Id: I3fa3b4f7e32531d48dfbb0ef946ff351ed5d9171