improve error message when missing raw AOs
Before:
> rivet-merge -e -o QCD3_0p3_nlo.yoda.gz files/OUTPUT_9992.yoda.gz files/OUTPUT_32656.yoda.gz
Merging data file files/OUTPUT_32656.yoda.gz [2/2]
Rivet.AnalysisHandler: INFO Rerunning finalize ...
libc++abi: terminating with uncaught exception of type std::out_of_range: vector
zsh: abort rivet-merge -e -o QCD3_0p3_nlo.yoda.gz files/OUTPUT_9992.yoda.gz
After:
> rivet-merge -e -o QCD3_0p3_nlo.yoda.gz files/OUTPUT_9992.yoda.gz files/OUTPUT_32656.yoda.gz
Rivet.AnalysisHandler: WARN No AOs selected from file: files/OUTPUT_9992.yoda.gz
Rivet.AnalysisHandler: WARN No AOs selected from file: files/OUTPUT_32656.yoda.gz
Insuffient pre-finalize AOs to do a reentrant run!