Better handling of plugin incompatibility

If a PHP is raised while executing plugin hook, Shaarli will display an error instead of rendering the error page (or just ending in fatal error for default hooks).
Also added phpErrorHandler which is handled differently that regular errorHandler by Slim.:

tests/plugins/test/test.php

@@ -19,3 +19,8 @@ function hook_test_random($data)
 
     return $data;
 }
+
+function hook_test_error()
+{
+    new Unknown();
+}