download: make DEM download failure fatal

As noted in [1], DEM download errors should be fatal.

[1] https://github.com/cuspaceflight/ruaumoko/pull/17/files#r19053619
download: properly wrap open() with a with statement

As noted in [1], relying on file objects from open() to be correctly
flished and closed on garbage collection is poor form.

[1] https://github.com/cuspaceflight/ruaumoko/pull/17/files#r19053639