Properly export errors while reading job list

In case of problems while reading the job list from disk (such as permission
errors) confd would silently fail, writing a warning on the log file but
sending an empty list and no error message to the client.

Also, tests have been updated in accordance to the new interface of the modified
functions.

This commit fixes this problem [Issue 405].

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>