Make SubstitutionError a subclass of ConfigurationError, so that every

exception specific to ZConfig is a specialization of
ConfigurationError.

doc/ZConfig/zconfig.tex

@@ -662,8 +662,7 @@ The following exceptions are defined:
 
 \begin{excdesc}{SubstitutionError}
   Base class for the specific exceptions raised by this module.
-  Instances provide the attribute \member{message}, which contains a
-  description of the error.
+  This is derived from \exception{ConfigurationError}.
 \end{excdesc}
 
 \begin{excdesc}{SubstitutionReplacementError}