refactor download script to allow for re-tries
refactor download script to allow for re-tries

- the EBI server can sometimes drop out, so we want to enable multiple
  tries if download is not successful
- read in contents of the metadata first then loop through the files
- split main loop into two separate loops
- set max tries to 10

dpastling/cnv_pipeline,dpastling/plethora,dpastling/plethora,dpastling/cnv_pipeline,dpastling/plethora