BB_NO_NETWORK can be set by bitbake internally by the use of
BB_FETCH_PREMIRRORONLY so update the error message to give users a
hint about this.
[YOCTO #3222]
(Bitbake rev: cac3060d0bf8c7deeacda18d06d92787911380d0)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>