Silent failures if implementations are not found

[kscaldef]

compare_problem_number and compare_implementations fail to notice if a language implementation is not found. Instead it just looks like that language runs really fast. E.g,

Benchmarking EP#002 with 100 iterations
            (warning: too few iterations for a reliable count)
                  Rate perl5/002/01.pl perl6/002/01.pl
perl5/002/01.pl 49.3/s              --            -79%
perl6/002/01.pl  233/s            372%              --