Catch and warn people importing MyEbirdData files after Excel editing

[Adam Winer]

Excel, annoyingly, re-formats date fields with an open-save cycle in CSVs. This generates broken imports, and confuses users.

Catch this explicitly (instead of allowing partial imports), and tell users “just import the whole thing, Scythebill can handle filtering”.

[Adam Winer]

Ditto any checklists

[Adam Winer]

Resolve Issue #447: Catch and warn people importing MyEbirdData files after Excel editing

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