Gladevcp: fix error on missing filter program

If an invalid filter program was specified in the INI file, the opened file
simply failed to load without warning except for a cryptic error on the command line.

lib/python/gladevcp/hal_filechooser.py

@@ -72,7 +72,7 @@ def update(self):
 
     def finish(self):
         # .. might be something left on stderr
-        for line in self.p.stderr:
+        for line in self.p.stderr.read().decode():
             m = progress_re.match(line)
             if not m:
                 self.stderr_text.append(line)