`ErrorCatcher` catches and shows tracebacks.  To test it we need a bad
application::

    >>> from simpleapp import simple_app
    >>> def bad_app(environ, start_response):
    ...     assert environ['REQUEST_METHOD'] == 'GET'
    ...     return simple_app(environ, start_response)

This application will raise ``AssertionError`` whenever the request
method is not ``GET``::

    >>> from testrunner import send_request
    >>> print send_request(bad_app, '/')
    200 OK
    ...
    >>> print send_request(bad_app, '/', method='POST')
    Traceback (most recent call last):
        ...
    AssertionError

Now lets wrap it in `ErrorCatcher` and retry those examples...

::

    >>> from errorcatcher import ErrorCatcher
    >>> wrapped_app = ErrorCatcher(bad_app)
    >>> print send_request(wrapped_app, '/')
    200 OK
    ...
    >>> print send_request(wrapped_app, '/', method='POST')
    500 Server Error
    Content-Type: text/plain
    Traceback (most recent call last):
    ...