Avoid keeping sending queries to an invalid nameserver

When a nameserver is known to be not workable due to wrong setup,
e.g. invalid address or network unreachable, it should be considered
unusable, just like what the code does for a nameserver which always
times out.

This change is beneficial to:
  [1] Fix the bug which the code can never mark all of the
      nameservers as unusable in some edge cases.
  [2] Decrease the noise of internal_error in the metrics.

Bug: 144828038
Test: cd packages/modules/DnsResolver && atest
Change-Id: Id84c49690a7ae1dbd209f9f7751120052efd6c13