adb: fix forward --list, --remove(-all) with multiple devices.

Also, fix error reporting when forward fails because we can't get a
transport, because there's either zero, or more than one transport that
matches the request.

Bug: http://b/111021517
Bug: http://b/111374366
Change-Id: Ia4c3bf6215c3ff4e9023ba1af556f5b10463bd6d
Test: manual