More robust calculation of median coverage
More robust calculation of median coverage

Catch errors when we can't calculate coverage on samples and avoid
trying to estimate high depth regions. Thanks to @ssaif

a113n/bcbio-nextgen,biocyberman/bcbio-nextgen,mjafin/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,vladsaveliev/bcbio-nextgen,biocyberman/bcbio-nextgen,guillermo-carrasco/bcbio-nextgen,vladsaveliev/bcbio-nextgen,biocyberman/bcbio-nextgen,lpantano/bcbio-nextgen,gifford-lab/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,mjafin/bcbio-nextgen,brainstorm/bcbio-nextgen,chapmanb/bcbio-nextgen,mjafin/bcbio-nextgen,lbeltrame/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,chapmanb/bcbio-nextgen,lpantano/bcbio-nextgen,gifford-lab/bcbio-nextgen,a113n/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,vladsaveliev/bcbio-nextgen,brainstorm/bcbio-nextgen,chapmanb/bcbio-nextgen,guillermo-carrasco/bcbio-nextgen,gifford-lab/bcbio-nextgen,lpantano/bcbio-nextgen,guillermo-carrasco/bcbio-nextgen,brainstorm/bcbio-nextgen,lbeltrame/bcbio-nextgen