Fix #73 Throw PugException only if facade/ignition is installed

Fix Laravel < 6 errors display compatibility

src/PugHandlerTrait.php

@@ -235,7 +235,7 @@ public function compileWith($path, callable $callback = null)
                     serialize($importCarrier->getCurrentImportPaths())
                 );
 
-                if ($pug->getOption('debug')) {
+                if ($pug->getOption('debug') && class_exists('Facade\\Ignition\\Exceptions\\ViewException')) {
                     $contents = "<?php try { ?>$contents<?php } " .
                         "catch (\Throwable \$exception) { throw new \Bkwld\LaravelPug\PugException(\$this, \$exception); }";
                 }