Fix comparison table when a test has failed

Reviewing the latest GitLab Versions comparsion performance test results we noticed that the git_push test was failing on older versions. When this happens the comparison table generation script isn't handling it gracefully and shows bad data:

NAME	11-7-0	11-8-0	11-9-0	11-10-0	11-11-0	12-0-0	12-1-0	12-2-0	12-3-0
git_push	ms (-)	ms (NaN%)	ms (NaN%)	ms (NaN%)	ms (NaN%)	ms (NaN%)	1371.53ms (+Infinity%)	1248.50ms (-8.97%)	1072.45ms (-14.1%)

Task is to fix it so it gracefully shows failures like the above with just dashes.

Raw comparison test result files for testing - 12.9.0-comparisons.zip