Change assertion to print strings that are unexpectedly non-ASCII.

Author: aardvark179

src/main/java/org/truffleruby/core/rope/RopeOperations.java

@@ -142,7 +142,7 @@ public static LeafRope encodeAscii(String value, Encoding encoding) {
 
     /** Prefer this to {@code getBytes(StandardCharsets.US_ASCII)} */
     public static byte[] encodeAsciiBytes(String value) {
-        assert StringOperations.isAsciiOnly(value);
+        assert StringOperations.isAsciiOnly(value) : "String contained non ascii characters \"" + value + "\"";
 
         final byte[] bytes = new byte[value.length()];