Note where redirect is trying to go

Author: dstansby

doc/sphinxext/redirect_from.py

@@ -94,11 +94,15 @@ def run(self):
         domain = self.env.get_domain('redirect_from')
         current_doc = self.env.path2doc(self.state.document.current_source)
         redirected_reldoc, _ = self.env.relfn2path(redirected_doc, current_doc)
-        if (redirected_reldoc in domain.redirects
-                and domain.redirects[redirected_reldoc] != current_doc):
+        if (
+            redirected_reldoc in domain.redirects
+            and domain.redirects[redirected_reldoc] != current_doc
+        ):
             raise ValueError(
                 f"{redirected_reldoc} is already noted as redirecting to "
-                f"{domain.redirects[redirected_reldoc]}")
+                f"{domain.redirects[redirected_reldoc]}\n"
+                f"Cannot also redirect it to {current_doc}"
+            )
         domain.redirects[redirected_reldoc] = current_doc
         return []