PrismLangException.getMessage(): Make robust if ASTElement printing fails

If there is a problem in ASTElement.toString for the AST element
that is optionally attached to the exception, we catch the exception
and return the error message without it.

prism/src/prism/PrismLangException.java

@@ -64,8 +64,14 @@ public class PrismLangException extends PrismException
 	{
 		String msg = super.getMessage();
 		if (e == null) return msg;
-		String s = e.toString();
-		if (s.length() < MAX_ERR_STR) {
+		String s = null;
+		try {
+			s = e.toString();
+		} catch (Exception ex) {
+			// in case there is a problem converting the AST element to a string
+			// we ignore it
+		}
+		if (s != null && s.length() < MAX_ERR_STR) {
 			if (e.hasPosition()) msg += " (\"" + s + "\", " + e.getBeginString() +")";
 			else msg += " (\"" + s + "\")";
 		}