Bug: importer should not store mails that don't have a Date: header

Related to #436, at present the importer uses the current timestamp if the mail has no Date: header.
Any such mail will be filed incorrectly.

The importer should report such mails and ignore them instead (as is done for some other mails that cannot be stored).

Maybe there should be an option to abort the import instead.

Start by tracing the importer path that handles messages without a Date: header, and compare it with the existing handling for mails that cannot be stored. Verify how import errors are reported and confirm that a mail lacking Date is reported and ignored rather than assigned the current timestamp; the optional abort behavior is left unresolved by the issue.

Written by the indexing model from the issue text.