Add error response during map load

[rchampine]

These two Syracuse volumes refuse to load. It is possible to click "Load Volume" but the sheets aren't populating.
https://oldinsurancemaps.net/loc/sanborn06296_015/
https://oldinsurancemaps.net/loc/sanborn06296_016/

[mradamcox]

From the load log:

  File "..../ohmg/loc_insurancemaps/models.py", line 395, in make_sheets
    sheet_no=parsed.sheet_number,
AttributeError: 'LOCParser' object has no attribute 'sheet_number'

Need to look into the parser, either something got changed inadvertently or there is something abnormal about these particular records. Hopefully can get to it this evening or sometime tomorrow.

[rchampine]

Interesting — no rush on our end. There are plenty of other volumes we can work on.

[mradamcox]

Looks like there is a missing page on these two, which is causing the error. Should have better handling on the ingestion side for this, so I'll leave this ticket open for now.