Clarify template error message (#1209)

youtube_dl/YoutubeDL.py

@@ -264,7 +264,7 @@ class YoutubeDL(object):
             self.report_error(u'Erroneous output template')
             return None
         except ValueError as err:
-            self.report_error(u'Insufficient system charset ' + repr(preferredencoding()))
+            self.report_error(u'Error in output template: ' + str(err) + u' (encoding: ' + repr(preferredencoding()) + ')')
             return None
 
     def _match_entry(self, info_dict):