No output if variable missing

If a variable is not present in one or both of the input files, Glance doesn't emit any message at all, silently skipping it. This is true for glance stats and I suspect other commands as well.

If the variable is only present in one of the two files, I would expect an warning message something like:

Skipping VARNAME: present in file1.nc but not file2.nc

If the variable isn't in either:

Skipping VARNAME: not present in either file.

The same for glance inspectStats when the only file does not contain the named variable.