Raise an error if an unknown preset is used to create a room. (#10738)

Raises a 400 error instead of a 500 if an unknown preset is passed
from a client to create a room.

changelog.d/10738.misc

@@ -0,0 +1 @@
+Additional error checking for the `preset` field when creating a room.

synapse/handlers/room.py

@@ -909,7 +909,12 @@ class RoomCreationHandler(BaseHandler):
             )
             return last_stream_id
 
-        config = self._presets_dict[preset_config]
+        try:
+            config = self._presets_dict[preset_config]
+        except KeyError:
+            raise SynapseError(
+                400, f"'{preset_config}' is not a valid preset", errcode=Codes.BAD_JSON
+            )
 
         creation_content.update({"creator": creator_id})
         await send(etype=EventTypes.Create, content=creation_content)