GEOT-4830 add fallback mark to SLD Graphic render

This fix applies to any 'failed' ExternalGraphics (unreachable at the
provided URI) and prevents nothing from being rendered as well as many
(one per feature) log messages and attempts to resolve the URI.

Additionally, the failed attempt is logged (once per render) as a
warning and contains the URI.
Merge pull request #489 from ischneider/GEOT-4380

 GEOT-4830 add fallback mark to SLD Graphic render