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.

[mradamcox]

To complete this, the load process needs to set "load error" as the map status when an error has occurred, and the frontend needs to be updated to look for this status and render the page accordingly.