system/timezone: check localtime exists

When installing a localtime, check it is a valid timezone.

[Peter: extend error message to make it clear to the user what to change]
Reported-by: Sagaert Johan <sagaert.johan@skynet.be>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>