Hardcode printing of "flowtext" warning to stderr

Third-party applications obviously can not handle additional output on stdout nor can they be expected to do any weird stdout/sterr redirection as we do via `options.stdout`
We probably shouldn't print anything in `scourString()` to start with unless we offer an option to disable all non-SVG output for third-party libraries to use.

scour/scour.py

@@ -3328,7 +3328,7 @@ def scourString(in_string, options=None):
         if options.error_on_flowtext:
             raise Exception(errmsg)
         else:
-            print("WARNING: {}".format(errmsg), file=options.ensure_value("stdout", sys.stdout))
+            print("WARNING: {}".format(errmsg), file=sys.stderr)
 
     # remove descriptive elements
     removeDescriptiveElements(doc, options)