smaller changes to the exceptions

src/Exceptions/LexerException.java

 package Exceptions;
 
 public class LexerException extends Exception {
+    private String message;
+
     public LexerException(String message) {
         super(message);
+        this.message = message;
+    }
+
+    @Override
+    public String toString() {
+        return "lexer failed: " + message;
     }
 }