Make error message about non downloading-file more informative.
Make error message about non downloading-file more informative.

at7/ensembl,at7/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,muffato/ensembl,mjg17/ensembl,at7/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,willmclaren/ensembl,danstaines/ensembl,muffato/ensembl,danstaines/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,muffato/ensembl,mjg17/ensembl,at7/ensembl,muffato/ensembl,mjg17/ensembl,Ensembl/ensembl,willmclaren/ensembl,Ensembl/ensembl,Ensembl/ensembl