Pre-flight check

I'm going to list some of the things I've added to or wanted on the IOL migrations.

First up is a pre-flight check which (for an XML file based import at least) first verifies the integrity of the import files before we even begin the import. It would loop over every import file and run it through `simplexml_load_file()` to see if any errors occur. For other import types, it would run them through `json_decode()` etc as appropriate.

On the QuiFinanza import we have a bunch of files without a BOM which is tripping up `simplexml_load_file()`, but I wasn't aware of this until the initial import was in progress.

Review the XML import path and how import files are selected; compare the proposed simplexml_load_file() and json_decode() checks for each supported import type. Done means invalid files are reported before the import begins, including the XML/BOM case described.

Written by the indexing model from the issue text.