Better diagnostics if no valid signature found

I downloaded Nix tonight, and immediately broke it by accidentally removing the default binary caching.
After figuring this out, I also failed to fix it properly, due to using the wrong key for Nix's default binary cache

If the diagnostic message would have been clearer about what/where a "signature" for a "substituter" is + comes from, it probably would have saved me a few hours.
Maybe we can save other noobs the same pain?

src/libstore/build/substitution-goal.cc

@@ -140,6 +140,8 @@ void PathSubstitutionGoal::tryNext()
     {
         warn("substituter '%s' does not have a valid signature for path '%s'",
             sub->getUri(), worker.store.printStorePath(storePath));
+        warn("verify that your nix.conf contains a correct signature in 'trusted-public-keys' for %s",
+            sub->getUri());
         tryNext();
         return;
     }