Revise section "Exceptions".

Beginning with Zope 4, the processing of exceptions has been changed
entirely, as the default publisher switched to WSGI.

**Note about line length**
ReStructered Text does not allow a line breakn with a bullet list
=> line length exceeded in favor of readabilty in rendered document.

modified:   docs/zdgbook/ObjectPublishing.rst

docs/zdgbook/ObjectPublishing.rst

@@ -926,42 +926,56 @@ button will also deactivate all other radio buttons from the other records.
 Exceptions
 ----------
 
-Unhandled exceptions are caught by the object publisher and are
-translated automatically to nicely formatted HTTP output.
-
-When an exception is raised, the exception type is mapped to an HTTP
-code by matching the value of the exception type with a list of
-standard HTTP status names. Any exception types that do not match
-standard HTTP status names are mapped to "Internal Error" (500). The
-standard HTTP status names are: "OK", "Created", "Accepted", "No
-Content", "Multiple Choices", "Redirect", "Moved Permanently", "Moved
-Temporarily", "Not Modified", "Bad Request", "Unauthorized",
-"Forbidden", "Not Found", "Internal Error", "Not Implemented", "Bad
-Gateway", and "Service Unavailable". Variations on these names with
-different cases and without spaces are also valid.
-
-An attempt is made to use the exception value as the body of the
-returned response. The object publisher will examine the exception
-value. If the value is a string that contains some white space, then
-it will be used as the body of the return error message. If it
-appears to be HTML, the error content type will be set to 'text/html',
-otherwise, it will be set to 'text/plain'. If the exception value is
-not a string containing white space, then the object publisher will
-generate its own error message.
-
-There are two exceptions to the above rule:
-
-1. If the exception type is: "Redirect", "Multiple Choices" "Moved
-   Permanently", "Moved Temporarily", or "Not Modified", and the
-   exception value is an absolute URI, then no body will be provided
-   and a 'Location' header will be included in the output with the
-   given URI.
-
-2. If the exception type is "No Content", then no body will be
-   returned.
-
-When a body is returned, traceback information will be included in a
-comment in the output.
+When the object publisher catches an unhandled exception, it tries to
+match it with a set of predifined exceptions coming from the
+**zExceptions** package, such as **HTTPNoContent**, **HTTPNotFound**,
+**HTTPUnauthorized**.
+
+If there is a match, the exception gets upgraded into the matching
+**zException**, which then results in a proper response returned to the
+browser, including a HTTP status code.
+
+.. note::
+
+     For a full list of exceptions please directly refer to the
+     implemented exception classes within the
+     `zExceptions package
+     <https://github.com/zopefoundation/zExceptions/blob/master/src/zExceptions/__init__.py>`_.
+
+
+.. attention::
+
+    When you create a custom exception, please make sure not to inherit
+    from **BaseException**, but e.g. from **Exception**, otherwise you'll
+    run into an waitress exception.
+
+.. note::
+
+    Beginning with Zope 4, a standard installation no longer comes with
+    a ``standard_error_message``.
+
+    There are two ways to catch and render an exception:
+
+    - create a ``standard_error_message``, which can be a **DTML Method**, **DTML Document**, **Python Script** or **Page Template**
+    - create an ``exception view``, see blog post `Catching and rendering exceptions <https://blog.gocept.com/2017/10/24/zope4-errorhandling/>`_
+
+If the exception is not handled, it travels up the WSGI stack.
+
+What then happens depends entirely on the WSGI middleware. By default
+Zope comes with **waitress** and by default **waitress** returns an error
+message as follows::
+
+  Internal Server Error
+
+  The server encountered an unexpected internal server error
+
+  (generated by waitress)
+
+.. note:: **Further information:**
+
+    `Debugging Zope applications under WSGI
+    <https://zope.readthedocs.io/en/latest/wsgi.html#debugging-zope-applications-under-wsgi>`_
+
 
 Exceptions and Transactions
 ---------------------------