Fix failing large note imports

When importing to Vaultwarden (or Bitwarden) notes larger then 10_000
encrypted characters are invalid. This because it for one isn't
compatible with Bitwarden. And some clients tend to break on very large
notes.

We already added a check for this limit when adding a single cipher, but
this caused issues during import, and could cause a partial imported
vault. Bitwarden does some validations before actually running it
through the import process and generates a special error message which
helps the user indicate which items are invalid during the import.

This PR adds that validation check and returns the same kind of error.
Fixes #3048