Unhelpful error on badly declared API export

Recently, didrocks & I were trying to expose GPG keys via the API.

As part of an early attempt, we accidentally said that IPerson.gpgkeys was a list of IGPGKeySets rather than IGPGKeys.

That is, we wrote:
    gpg_keys = exported(
        CollectionField(
            title=_("List of valid OpenPGP keys ordered by ID"),
            readonly=False, required=False,
            value_type=Reference(schema=IGPGKeySet)))

Rather than:
    gpg_keys = exported(
        CollectionField(
            title=_("List of valid OpenPGP keys ordered by ID"),
            readonly=False, required=False,
            value_type=Reference(schema=IGPGKey)))

When we tried to test this (a step which demanded building the wadl & apidocs first), we got an obsure error: ` AssertionError: No IEntry adapter found for IGPGKeySet (web service version: devel).`

The error gave no hint of which declaration was causing the problems, not even which file the bad declaration was in, so we spent a lot of time trying to expose IGPGKeySet as a collection.

It would be very useful if the error message included some reference to the declaration that causes the problems. Bonus points for not being an assertion error.