Fail gracefully when kroki.io misbehaves #41
Description
We currently depend on kroki.io for rendering the output of EGX examples that generate PlantUML or Graphviz (e.g. https://eclipse.dev/epsilon/playground/?wbs2puml). When kroki.io misbehaves or is down, we don't produce an error message in the output panel (it is just left blank), which can be confusing for users. In such cases we should report a relevant error message and show the generated PlantUML/Graphviz text in the ouput panel instead.