Add correct M_BAD_JSON error code to /profile/{userId}/displayname (#8517)

Fixes #8029

changelog.d/8517.bugfix

@@ -0,0 +1 @@
+Fix error code for `/profile/{userId}/displayname` to be `M_BAD_JSON`.

synapse/rest/client/v1/profile.py

@@ -59,7 +59,9 @@ class ProfileDisplaynameRestServlet(RestServlet):
         try:
             new_name = content["displayname"]
         except Exception:
-            return 400, "Unable to parse name"
+            raise SynapseError(
+                code=400, msg="Unable to parse name", errcode=Codes.BAD_JSON,
+            )
 
         await self.profile_handler.set_displayname(user, requester, new_name, is_admin)