Be more strict about RRFS-CMAQ reader PM2.5 merge

[zmoon]

Fixes #78

• test of this exception handling

[zmoon]

The Python 3.6 run here was getting xarray 0.18.2, which has a different message in this situation. But now accounted for.

[zmoon]

@rschwant if you can take a quick look at some point that would be great

[zmoon]

Suggested change

	"""
	`right_name` is used for context in the error messages if provided.
	"""

[rschwant]

This looks good to me. I like the detail of the error messages, so that it is really clear when something goes wrong. Was Jianping able to fix the float issue in his scripts, so that these files can merge properly now?

[zmoon]

Thanks @rschwant

Was Jianping able to fix the float issue in his scripts, so that these files can merge properly now?

I haven't gotten an update on that. I had the same issue with some RRFS-CMAQ output from Patrick, and I did some pre-processing using ds = ds.merge(ds_pm, join="exact", compat="override"), which afaik at least checks that the index shapes match.