In the nextjs SDK, when a user hits the _error page, the original transaction name gets overwritten with _error. This causes two problems:

1. It makes it so a user would not be able to tell that they have a particularly buggy route, since every time it errored, it would show up in the Sentry UI filed under _error rather than the real route name.

2. The DSC can get propagated with the original transaction name, but that then doesn't match up with the eventual transaction name.

We should prevent the original name from getting overwritten.

(This is current as of 7.13.0.)