Reset content-length for unhandled exceptions

This prevents `ProtocolError` caused by the error middleware not being
able to reset the content length. Only affects routes that explicitly
set the content-length and don't have a default response handler.

This is effectively the case when using `web.expose`. To test this,
make sure you get Galaxy's error middleware output in the browser
and your console.
```
diff --git a/lib/galaxy/webapps/galaxy/controllers/dataset.py b/lib/galaxy/webapps/galaxy/controllers/dataset.py
index c95d9df788..92b04684d3 100644
--- a/lib/galaxy/webapps/galaxy/controllers/dataset.py
+++ b/lib/galaxy/webapps/galaxy/controllers/dataset.py
@@ -456,6 +456,8 @@ class DatasetInterface(BaseUIController, UsesAnnotations, UsesItemRatings, UsesE
     def imp(self, trans, dataset_id=None, **kwd):
         """Import another user's dataset via a shared URL; dataset is added to user's current history."""
         # Set referer message.
+        trans.response.headers["content-length"] = 0
+        trans = None
         referer = trans.request.referer
         if referer and not referer.startswith(f"{trans.request.application_url}{url_for('/login')}"):
             referer_message = f"<a href='{escape(referer)}'>return to the previous page</a>"

```

lib/galaxy/web/framework/base.py

@@ -264,6 +264,7 @@ def handle_request(self, request_id, path_info, environ, start_response, body_re
         except Exception as e:
             body = self.handle_controller_exception(e, trans, method, **kwargs)
             if not body:
+                trans.response.headers.pop("content-length", None)
                 raise
         body_renderer = body_renderer or self._render_body
         return body_renderer(trans, body, environ, start_response)