Add user advice to UnableToDetermineCurrentEpoch error.

Ideally, we'd be able to give the user some indication of how long they
should wait, but assuming we cannot reliably know this information,
letting them know that waiting may resolve the issue is arguably better
than giving no advice at all.

lib/core/src/Cardano/Wallet/Api/Server.hs

@@ -1997,8 +1997,10 @@ showT = T.pack . show
 instance LiftHandler ErrCurrentEpoch where
     handler = \case
         ErrUnableToDetermineCurrentEpoch ->
-            apiError err500 UnableToDetermineCurrentEpoch
-                "I'm unable to determine the current epoch."
+            apiError err500 UnableToDetermineCurrentEpoch $ mconcat
+                [ "I'm unable to determine the current epoch. "
+                , "Please wait a while for the node to sync and try again."
+                ]
 
 instance LiftHandler ErrUnexpectedPoolIdPlaceholder where
     handler = \case