adb: don't error in handle_host_request if we can't acquire a transport.

Host services are attempted after handle_host_request, which means that
failing to find a transport to give to handle_forward_request shouldn't
send an error over to the other end.

Bug: http://b/78294734
Test: `adb track-devices` with multiple devices connected
Change-Id: I46c89cc1894b51d48fea7d4e629b1d57f73e3fd6