Export the Neo4jGraphQLError classes

[AccsoSG]

Is your feature request related to a problem? Please describe.
Currently, the Neo4jGraphQLError classes cannot be imported into the own project.
However, it would be useful to import these classes for custom error handling.

Describe the solution you'd like
Modify the index.ts so that the error classes are also exported.

Additional context
Specifically, I would like to be able to react to the Neo4jGraphQLErrors in the request lifecycle in the Apollo server. An example would be a special logging, or a change in the HTTP status code when a certain Neo4jGraphQLError occurs.