export-login --expires silently ignores dates given in ISO 8601 format

[Björn Tillenius]

If you pass the data in ISO 8601 format to export-login --expires:

  snapcraft export-login --expires 2022-01-01 --snaps maas --acls package_release macaroon

You get a macaroon with expiry of None:

  Login successfully exported to 'macroon'. This can now be used with

      snapcraft login --with macaroon

  to log in to this account with no password and have these capabilities:
  snaps: ['maas']
  channels: No restriction
  permissions: ['package_release']
  expires: None

If I pass the date, including the time, it works. But the documentation for export-login says that a date should be enough.

[Sergio Schvezov (sergiusens)]

We should probably at least error on dates that could not be parsed