Review comments

proxy/proxy.go

@@ -812,14 +812,13 @@ func (c *client) interceptSystemQuery(hdr *frame.Header, stmt interface{}) {
 		}
 	case *parser.UseStatement:
 		if _, err := c.proxy.maybeCreateSession(hdr.Version, s.Keyspace); err != nil {
+			errMsg := "Proxy unable to create new session for keyspace"
 			var cqlError *proxycore.CqlError
-			switch {
-			case errors.As(err, &cqlError):
-				errMsg := cqlError.Message
-				c.send(hdr, errMsg)
-			default:
-				c.send(hdr, &message.ServerError{ErrorMessage: "Proxy unable to create new session for keyspace"})
+			if errors.As(err, &cqlError) {
+				// copy detailed error reason from downstream message
+				errMsg = cqlError.Message.GetErrorMessage()
 			}
+			c.send(hdr, &message.ServerError{ErrorMessage: errMsg})
 		} else {
 			c.keyspace = s.Keyspace
 			// We might have received a quoted keyspace name in the UseStatement so remove any