Improve substitution conflict error message

Fixes #1715

Author: Regan-Koopmans

substratevm/src/com.oracle.svm.hosted/src/com/oracle/svm/hosted/substitute/AnnotationSubstitutionProcessor.java

@@ -819,11 +819,13 @@ private static boolean isIncluded(TargetElement targetElementAnnotation, Class<?
 
     private static <T> void register(Map<T, T> substitutions, T annotated, T original, T target) {
         if (annotated != null) {
-            guarantee(!substitutions.containsKey(annotated) || substitutions.get(annotated) == original || substitutions.get(annotated) == target, "Already registered: %s", annotated);
+            guarantee(!substitutions.containsKey(annotated) || substitutions.get(annotated) == original || substitutions.get(annotated) == target,
+                            "Substition: %s conflicts with previously registered: %s", annotated, substitutions.get(annotated));
             substitutions.put(annotated, target);
         }
         if (original != null) {
-            guarantee(!substitutions.containsKey(original) || substitutions.get(original) == original || substitutions.get(original) == target, "Already registered: %s", original);
+            guarantee(!substitutions.containsKey(original) || substitutions.get(original) == original || substitutions.get(original) == target,
+                            "Substition: %s conflicts with previously registered: %s", original, substitutions.get(original));
             substitutions.put(original, target);
         }
     }