Improve error msg in DNBProcessAttach if we can't stop inferior

When we attach to a process, we task_for_pid(), ptrace(), and then
we try to halt execution of the process and time out after thirty
seconds if we cannot interrupt it.  At this point, we must assume
we have no control of the inferior process and the attach has failed.
The error message we return currently is "operation timed out"; this
change improves on that error message to make it more clear what is
going on.  Thanks to Jim for pointing this out.

rdar://101152233