Handle missing graphviz apps more elegantly in output

[mulkieran]

The graphviz extension uses the graphviz applications, specifically at least, dot to generate a graphics file from the output of other extensions, .e.g., inheritance_diagram. If dot is missing, then instead of the image, the actual dot output of the other extension is displayed in the generated html.

It would be nice if there was a pretty way of handling the missing dot app, like:

• stowing the dot output in a separate file
• using a default image in the place of the one that can't be generated, perhaps with a link to the dot file, and text something like: Failed to generate image file from source, missing application dot. See intermediate dot output at link: <url>

This would work well with a workflow where the dot output was generated anyhow into a separate file, and also, I think, the suggestion in #655.

[GadgetSteve]

One suggestion might be to allow the setting of an alternate dot tool such as one of the on-line graphviz processing tools, e.g.: http://www.webgraphviz.com/ and extending the graphviz extension to include using it if a local graphviz dot tool is not found. I personally would suggest having a configuration tag graphviz_allow_web so that the user has the option of enabling it.