new: [compose-core] provide full set of ``p-err`` and ``read-0-err`` for charms

bin/compose-core

@@ -136,6 +136,13 @@ read-0-err() {
         false
     }
 }
+export -f read-0-err
+
+p-err() {
+    "$@"
+    echo "$?"
+}
+export -f p-err
 
 
 wyq() {