ability to customize errors thrown by ent

[lolopinto]

a smaller step to https://github.com/lolopinto/ent/issues/1224?

when ent throws exceptions, it throws an instance of https://github.com/lolopinto/ent/blob/93987f60cd5c5d005945d25bd73afb00f076d0e8/ts/src/core/privacy.ts#L27-L48

which could include the whole ent which has lots of stuff in ent and sometimes circular dependencies which JSON.stringify doesn't like if we're sending the error back to the client