Image is created dispite of errors but not displayed

[Potherca]

Sometimes when an error is returned by dot the graph image is still created.
When the same data is submitted again the image generated by the previous submit is actually shown.

Instead of trusting Graphviz error codes it might be saner to just check if an image file was generated and base error output on that validation.

The only question that remains is what to do with the errors from the previous run?
We could save the generated output in a log file and include that with any following commit of the same data, instead of just saying "file already exists"