Do not fail subsequent jobs if one doesn't have associated gpubox files

[harrisonbarlow]

If we ask it to download a series of observation but one of them doesn't have any associated gpubox fits files, all the subsequent downloads also fail. An example is this (ultra-short) list:
1279803256
1283772224
The first has no files and the second does have files, but if you ask mwa_client for both then you will get neither.

[tjgalvin]

I have made #32 that should do the trick. On my particular test file it skips the ~50 or so observations that have no gpufits files and will continue on the remaining ones that do.