Upload rarely fails, most often it just cannot read some files it generated.
Rerun helps, but takes an hour. And if it's happening on the master branch, the badge show failing until either someone reruns codecov in travis (and it passes) or a new commit is pushed to master (and everything passes).

E.g. https://travis-ci.org/seqan/seqan3/jobs/558817859

WARNING: negative counts found in tracefile seqan3_coverage.captured
ERROR: cannot write to seqan3_coverage.total!

15.07.2019

We try to use absolute paths, since there is an issue with relative paths in the lcov version we use.