legal-info: fail trying to copy a non-existent license file

If a package's _LICENSE_FILES contains the name of a non-existent file,
the make process would continue and return true, unless the
non-existent file is the last listed.

Fix this wrong beaviour by failing with an error when any of the listed
files is missing.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>