dispute-coordinator: Make handle_import_statements return FatalResult

[eskimor]

instead of Result here.

Reason being: We for example increase spam slots in this function, if then the import fails for some non fatal and unrelated reason, we should likely actually decrement previously incremented spam slots again, for non fatal errors - which is cumbersome and actually not needed, as in practice this function only has fatal errors right now, but it is not properly marked as such, because some used db related functions are not properly returning FatalResult yet.

We should therefore:

• let handle_import_statements return FatalResult instead of Result (by fixing called functions to do the same).
• Add documentation, why this function should only return FatalResult

[eskimor]

@BradleyOlson64 that's an easy/small one.