Adds a standardised Error field to the fields package

Usage:

log.Info("I have a bad feeling about this", fields.Error, err)

Outcome

We've got a number of different fields within our logging setup that all roughly equate to errors:

• json.api_error
• json.extra.error
• json.error_message
• json.error_type
• json.extra.ai_gateway_error_detail
• json.telesign_errors

As well as a number of json.graphql_errors.* fields.

This MR should define the consistent way for us to log out errors across our system.