Fix color output for non-256 color terminals

The function enableVirtualTerminalProcessing must return an error if
virtual terminal processing (only applicable on Windows) was not
enabled, otherwise we assume that the terminal supports both 256-color
and truecolor.

We have been erroneously assuming that all non-Windows terminals are
256-color and sending escape sequences to those that cannot intepret
them. This led to some parts of Survey prompts being invisible on old
terminals.

Regressed in be4b392530

pkg/iostreams/console.go

@@ -3,6 +3,8 @@
 
 package iostreams
 
+import "errors"
+
 func enableVirtualTerminalProcessing(fd uintptr) error {
-	return nil
+	return errors.New("not implemented")
 }