Clean up gz* function return values.

In some cases the return values did not match the documentation,
or the documentation did not document all of the return values.
gzprintf() now consistently returns negative values on error,
which matches the behavior of the stdio fprintf() function.

gzlib.c

@@ -615,7 +615,6 @@ void ZLIB_INTERNAL gz_error(state, err, msg)
     strcat(state->msg, ": ");
     strcat(state->msg, msg);
 #endif
-    return;
 }
 
 #ifndef INT_MAX