BUG: RAMSESDataset fails to load

[neutrinoceros]

Bug report

Bug summary

This was reported on the mailing list https://mail.python.org/archives/list/yt-users@python.org/thread/5CMIDJGXF452PX6IGBGAGMAE3RKEQYD3/

So I think there are at least two different problems there:

• the error message is malformed
• it is not informative enough (there are a couple reasons why RAMSESDataset._is_valid can be False, but here we're swallowing everything)
• I guess the data should actually load, but there is not enough information in the original report to assess what's actually going on

I'll link this issue to the person who reported, so we can work together on the last point. Meanwhile, I can fix the first two on my own

[neutrinoceros]

Taking this out of the milestone for now. The first bugfix is ready so it can easily be back ported, but the underlying problem has no solution yet so it shouldn't block a release.

[neutrinoceros]

Andrews Wright uploaded the dataset here:
http://use.yt/upload/a4d14d4e