add TestRenterDownloadError to verify failed download calls also set the Error field
add TestRenterDownloadError to verify failed download calls also set the
Error field

ericflo/Sia,ericflo/Sia,NebulousLabs/Sia,mharkus/Sia,ChrisSchinnerl/Sia,MrStrong/Sia,MrStrong/Sia,NebulousLabs/Sia,mtlynch/Sia,mharkus/Sia,mtlynch/Sia,ChrisSchinnerl/Sia