Print full tracebacks from exceptions when debugging is enabled.

av98.py

@@ -32,6 +32,7 @@ import subprocess
 import sys
 import tempfile
 import time
+import traceback
 import urllib.parse
 import uuid
 import webbrowser
@@ -382,6 +383,7 @@ you'll be able to transparently follow links to Gopherspace!""")
 Slow internet connection?  Use 'set timeout' to be more patient.""")
         else:
             ui_out.error("ERROR: " + str(err))
+            ui_out.debug(traceback.format_exc())
 
     def _handle_local_file(self, gi):
         if gi.path.endswith(".gmi"):    # TODO: be better about this