Improved error reporting when dropping features

Prints a more helpful error message, saying what features have been
dropped.

core/src/main/kotlin/se/lth/cs/util/Features.kt

@@ -204,8 +204,11 @@ class FeatureSet(vararg features : Feature) : ArrayList<Feature>() {
      * Returns a subset of features which should be supported by the current machine!
      */
     fun checked() : FeatureSet {
-        val featuresChecked = this.filter { it.isSupported() }
-        return FeatureSet(*featuresChecked.toTypedArray())
+        val featuresChecked = this.partition { it.isSupported() }
+
+        System.err.println("[WARNING]: Some features have been dropped, they are not supported on this machine: ${featuresChecked.second}")
+
+        return FeatureSet(*featuresChecked.first.toTypedArray())
     }
 
     fun accept(visitor : FeatureVisitor) {