This is a pretty significant restructuring of ZPublisher to
allow a new module hook for handling errors.  There are two goals
in this:

  - To allow Zope to retry a request when ZODB raises a ConflictError, and

  - To move reformatting of exception using standard_error_message
    out to the publisher level so that:

    - Errors don't get obfuscated by error handling done by
      sub-documents and external methods, and

    - Errors raised by the publisher (e.g. for input errors,
      not-found errors, system exit, etc) get prettified.