feat(config): warn when an account configures no trash folder

The trash key is mandatory: Delete moves a file into it, so an
account without one cannot delete at all. Report it as a config
problem naming the account and the key, rather than degrading
Delete silently, per the existing "a warning the user cannot act on
teaches them to ignore warnings" rule (item 83).

Several existing test fixtures loaded accounts with no trash key and
asserted zero problems/warnings; added trash=Trash to those where it
was incidental to what the test actually covers.

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