Improve error message in axisswap

The test for parameters in axisswap actually is a bit convoluted, but
in essence it checks both whether ANY of the 'axis' and 'order'
parameters are given, and whether BOTH are given.

Hence, the error message

"order and axis parameters are mutually exclusive"

is misleading in the case where none of the two is given.
The updated message:

"must provide EITHER 'order' OR 'axis' parameter"

is intended to cover both cases in an informative way

src/conversions/axisswap.cpp

@@ -170,7 +170,7 @@ PJ *PJ_CONVERSION(axisswap, 0) {
     if (!pj_param_exists(P->params, "order") ==
         !pj_param_exists(P->params, "axis")) {
         proj_log_error(P,
-                       _("order and axis parameters are mutually exclusive."));
+                       _("must provide EITHER 'order' OR 'axis' parameter."));
         return pj_default_destructor(
             P, PROJ_ERR_INVALID_OP_MUTUALLY_EXCLUSIVE_ARGS);
     }