Move RGS `GraphSyncError` into the top-level module

Move RGS `GraphSyncError` into the top-level module

The top-level module is only a few hundred lines, so there's not a
lot of reason to hide the `GraphSyncError` in its own module.
Instead, we simply move it to the top-level `lib.rs`, which doesn't
change the public API as it was previously re-exported at the top
level.