gdbserver/libthread_db: Don't ignore memory reading failures

gdbserver/libthread_db: Don't ignore memory reading failures

If we had this in place before, then the regression fixed by the
previous commit would have been been visible is all test runs.  E.g.:

  Running src/gdb/testsuite/gdb.threads/multi-create-ns-info-thr.exp ...
  FAIL: gdb.threads/multi-create-ns-info-thr.exp: continue to breakpoint 6

Debugging manually we'd see this:
  gdbserver: Cannot get thread handle for LWP 1467: generic error

Instead of:
  gdbserver: PID mismatch!  Expected 27472, got 27471

which is misleading - gdbserver didn't 27471, that was stale stack
data from previous function invocations.

gdb/gdbserver/ChangeLog:
2017-09-29  Pedro Alves  <palves@redhat.com>

	* proc-service.c (ps_pdread): Return PS_ERR if reading memory
	fails.