Add trace_offset to error handler

Using the trace_offset will allow wrapped errors to have the correct
code and context displayed.

Refs #718

Author: markstory

src/Plugin.php

@@ -87,6 +87,7 @@ function ($code, $message, $file, $line, $context = null) use (&$previousHandler
                         return;
                     }
                     if ($previousHandler) {
+                        $context['_trace_frame_offset'] = 1;
                         return $previousHandler($code, $message, $file, $line, $context);
                     }
                 }