Update index.rst

Make it easier for Intersphinx to point at the scrolled-down start of the doc.
Update index.rst

Dang, underscore.
render the exception a second time below traceback. especially template tracebacks are quite long sometimes, showing the exc below the traceback makes it easier to understand the context
see github issue #54. Introducing a new configuration option : debugtoolbar.show_on_exc_only = true/false. If true the debug toolbar will only be rendered if an exception occurs; if the request does not raise a exception no additional html code will be added to the response.
tests for new show_on_exc_only configuration option
updated contributors.txt
added show_on_exc_only configuration option update
added show_on_exc_only configuration option
avoid duplicate sections
use bytes for py3
more python3 fixes