poolmanager: Warn when spacecostfactor is too big

When space cost factor is large, wass may run into a problem caused by
the limited precision of floating point arithmetic. When that happens
the code reaches a statement that was thought to be unreachable.

This patch adds a check that detects the situation and generates
a more suitable error message.

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: https://rb.dcache.org/r/7399/
(cherry picked from commit b9b1ecd8ad31662da627d48aaafca367af164032)
Merge pull request #988 from gbehrmann/fix/2.9/rb7399

poolmanager: Warn when spacecostfactor is too big