Add stricter error checking
Add stricter error checking

We will first try to use the offset supplied by the configuration.
If that doesn't work we will try all other offset possibilities,
in priority from custom, oldest, and then newest. If we still
can't find a valid offset we will just crash.

replay/metrictank,raintank/raintank-metric,raintank/metrictank,replay/metrictank,replay/metrictank,raintank/metrictank,raintank/raintank-metric,raintank/metrictank,raintank/metrictank,replay/metrictank,raintank/raintank-metric