Provide error messaging when input datasets are unavailable

[vonw]

Excerpt from mail from Joe Vaughan on 7 January 2022:

"I recently discovered that we’d been failing to get WACCM for BCON for months!
I didn’t find that we had received proper notice of a change of the relevant url.
I fixed it, but this points out that AIRPACT doesn’t do a god job of self-documenting when it gets BCON from WACCM, and when it runs using prior day, or even fails over to very old monthly stuff."