oper.gi: add detail to DigraphDisjointUnion error message

Author: wilfwilson

gap/oper.gi

@@ -410,7 +410,7 @@ function(arg)
   if not IsList(arg) or IsEmpty(arg)
       or not ForAll(arg, IsDigraphByOutNeighboursRep) then
     ErrorNoReturn("the arguments must be digraphs by out-neighbours, or a ",
-                  "single list of digraphs by out-neighbours,");
+                  "single non-empty list of digraphs by out-neighbours,");
   fi;
 
   D := arg[1];

tst/standard/oper.tst

@@ -876,14 +876,14 @@ gap> gr2 := CompleteDigraph(100);
 gap> DigraphDisjointUnion(gr) = gr;
 true
 gap> DigraphDisjointUnion([[]]);
-Error, the arguments must be digraphs by out-neighbours, or a single list of d\
-igraphs by out-neighbours,
+Error, the arguments must be digraphs by out-neighbours, or a single non-empty\
+ list of digraphs by out-neighbours,
 gap> DigraphDisjointUnion([gr], [gr]);
-Error, the arguments must be digraphs by out-neighbours, or a single list of d\
-igraphs by out-neighbours,
+Error, the arguments must be digraphs by out-neighbours, or a single non-empty\
+ list of digraphs by out-neighbours,
 gap> DigraphDisjointUnion(gr, Group(()));
-Error, the arguments must be digraphs by out-neighbours, or a single list of d\
-igraphs by out-neighbours,
+Error, the arguments must be digraphs by out-neighbours, or a single non-empty\
+ list of digraphs by out-neighbours,
 gap> DigraphDisjointUnion(gr, gr);
 <immutable digraph with 2000 vertices, 2000 edges>
 gap> DigraphDisjointUnion([gr2, gr2]);