Neo4jError and ClientError deprecations and small adjustments

[robsdedude]

Slightly change Neo4jError and ClientError:

• Properties message and code are always a str (instead of str | None).
• Remove possibility to override/set message and code properties.
• Remove undocumented, internal methods Neo4jError.hydrate, Neo4jError.invalidates_all_connections,
  and Neo4jError.is_fatal_during_discovery.
• Remove deprecated method Neo4jError.is_retriable.
  Use Neo4jError.is_retryable instead.
• Change string representation of Neo4jError to include GQL error information.

[MaxAake]

💬

[MaxAake]

LGTM!