Make all exception classes inherit from std::exception

This refactors out a very common pattern of formatting "%s: %s" with
e.function.c_str() and strerror( e.the_errno ) into just the what()
method of NetworkException. It's also a prerequisite for making cleaner
public API for any exceptions we throw, and allows us to more easily
get exceptions passed back to us to handle.

src/frontend/stmclient.cc

@@ -568,7 +568,7 @@ void STMClient::main( void )
         throw;
       } else {
         wchar_t tmp[ 128 ];
-        swprintf( tmp, 128, L"Crypto exception: %s", e.text.c_str() );
+        swprintf( tmp, 128, L"Crypto exception: %s", e.what() );
         overlays.get_notification_engine().set_notification_string( wstring( tmp ) );
       }
     }