server: give more precise error message

There are two same error messages with different cause.
Let user know what is the cause of the error.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>