Clarify repo mismatch message (fixes #331)

model/model.go

@@ -279,8 +279,7 @@ func (m *Model) Index(nodeID string, repo string, fs []protocol.FileInfo) {
 	if r, ok := m.repoFiles[repo]; ok {
 		r.Replace(id, files)
 	} else {
-		l.Warnf("Index from %s for unexpected repo %q; verify configuration", nodeID, repo)
-
+		l.Warnf("Unexpected repository ID %q sent from node %q; ensure that the repository exists and that this node is selected under \"Share With\" in the repository configuration.", repo, nodeID)
 	}
 	m.rmut.RUnlock()
 }