support/mkusers: enhance error messages

The current error messages are a bit terse, and do not provide all the
required information.

Expand them to provide more context.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Tom Elliott <tommygunsster@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>