Defend reading data as xarray against quirks getting attributes from voview files

[takluyver]

Fixing an issue @vradorov encountered, at two levels:

• If there's an error reading the units in .xarray(), show a warning but still return the xarray without the units. The units attribute is nice to have, but it shouldn't break the method if we can't read it.
• The specific case here was with getting the data selection for the last sequence file - if we only read the filename from the first file, it doesn't hit the error.