[Task] Improve LOC parser

[mradamcox]

Describe the problem

The LOC API parser that is used during volume load needs to be improved to handle some edge cases I've encountered.

• Handle filelists that have two entries referencing the same file. See [Bug] Incomplete volume loads #197 for more info. There is an example of this in sanborn04023_023.
• Handle filelist entries that don't have a JP2 url in them. There is an example of this in item sanborn04339_029.

Expected behavior
Ideally, this parser should inspect the record before creating any objects in the database, and return an error if anything unexpected is found.