better error message on devnet mismatch

Change-Id: Ife724b6b4a3d18e840f98dc5adc141b206d48209

src/Chainweb/Version/Registry.hs

@@ -129,17 +129,21 @@ lookupVersionByCode code
         -- the case that we don't actually need the version, just the code.
         lookupVersion & versionCode .~ code
     where
+
     lookupVersion :: HasCallStack => ChainwebVersion
     lookupVersion = unsafeDupablePerformIO $ do
         m <- readIORef versionMap
         return $ fromMaybe (error notRegistered) $
             HM.lookup code m
+
     notRegistered
-        | code == _versionCode recapDevnet = "recapDevnet version used but not registered, remember to do so after it's configured"
-        | code == _versionCode devnet = "devnet version used but not registered, remember to do so after it's configured"
-        | code == _versionCode pact5Devnet = "Pact 5 devnet version used but not registered, remember to do so after it's configured"
+        | code == _versionCode recapDevnet = "recapDevnet version used but not registered, remember to do so after it's configured. " <> perhaps
+        | code == _versionCode devnet = "devnet version used but not registered, remember to do so after it's configured. " <> perhaps
+        | code == _versionCode pact5Devnet = "Pact 5 devnet version used but not registered, remember to do so after it's configured. " <> perhaps
         | otherwise = "version not registered with code " <> show code <> ", have you seen Chainweb.Test.TestVersions.testVersions?"
 
+    perhaps = "Perhaps you are attempting to run a different devnet version than a previous run, and you need to delete your db directory before restarting devnet with the new version?"
+
 -- TODO: ideally all uses of this are deprecated. currently in use in
 -- ObjectEncoded block header decoder and CutHashes decoder.
 lookupVersionByName :: HasCallStack => ChainwebVersionName -> ChainwebVersion