Expose error fields

[aljazerzen]

Currently, we do have property getters for error fields: https://github.com/edgedb/edgedb-python/blob/ca401141128dba2da443657edd61e45551e53b78/edgedb/errors/_base.py#L98-L127

... but they are not considered stable API. We should change that and also expose details field.

We do have similar code in the server:

https://github.com/edgedb/edgedb/blob/master/edb/errors/base.py#L178-L200

[msullivan]

I think probably details should be dumped in the default formatting