billing: Improve error message on failur to parse date arguments

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: yes
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: https://rb.dcache.org/r/7359/
(cherry picked from commit ea8b501786bff2c6bbaf026d0603e54d1923875a)
Merge pull request #909 from gbehrmann/fix/2.10/rb7359

billing: Improve error message on failur to parse date arguments