Refactored error message handling so that SimpleItem's raise_standardErrorMessage sniffs the response to see if it wants an html encoded error message or not.

If so, then standard_error_message is rendered as usual as the error value. Otherwise the error value text is passed through unchanged.

XML-RPC responses now request text/plain error format so that standard_error_message is not envoked to render the fault string. Because of this, the previous formatting code could be removed, although html tags are still stripped because error values themselves sometimes contain tags 8^/