Rna-transcription: throw exception in starter implementation

exercises/rna-transcription/src/main/java/RnaTranscription.java

@@ -1,3 +1,5 @@
 public class RnaTranscription {
-    public String ofDna(String dnaString) { return null; }
+    public String ofDna(String dnaString) {
+        throw new UnsupportedOperationException("Method has not been implemented yet.");
+    }
 }