sanity.bbclass: mention CONNECTIVITY_CHECK_URIS in network failure message

This expands the error message when a network failure is detected.
It happens that some ISPs or networks block the default example.com
domain. Therefore, instead of disabling network access, it
lets the user know how to modify the test URL.

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>