Protect pretty print against errors caused because a MathObject hasn'…

…t been properly stringified.

This doesn't solve the basic issue which is to make sure that objects are stringified before leaving the Safe compartment.  This does however make the error messages more useful when that happens.

lib/PGUtil.pm

@@ -92,7 +92,17 @@ sub pretty_print_html {    # provides html output -- NOT a method
     $level--;
     return "PGalias has too much info. Try \$PG->{PG_alias}->{resource_list}" if ref($r_input) eq 'PGalias';  # PGalias just has too much information
     return 'too deep' unless $level > 0;  # only print four levels of hashes (safety feature)
-    my $out = '';
+	my $out = '';
+	    # protect against modules defined in Safe which can't find their stringify procedure.
+		$r_input = $r_input//'';
+		eval { "$r_input" };
+		if ($@ ) {
+			$out = "Unable to determine stringify this item\n";
+			$out .= $@. "\n";
+			return ($out);
+		}
+
+
     if ( not ref($r_input) ) {
     	$out = $r_input if defined $r_input;    # not a reference
     	$out =~ s/</&lt;/g  ;  # protect for HTML output