[java] make the json parsing exception text more helpful

Author: joerg1985

java/src/org/openqa/selenium/json/Input.java

@@ -97,7 +97,21 @@ public String toString() {
       length = MEMORY_SIZE;
     }
 
-    return "Last " + length + " characters read: " + new String(buffer, offset, length);
+    String last = "Last " + length + " characters read: " + new String(buffer, offset, length);
+    int next = Math.min(MEMORY_SIZE, filled - (offset + length));
+
+    if (next > 0) {
+      if (next > 128) {
+        next = 128;
+      }
+      return last
+          + ", next "
+          + next
+          + " characters to read: "
+          + new String(buffer, offset + length, next);
+    }
+
+    return last;
   }
 
   /**