Add graphviz source code for error handling diagram

[nagashi]

Recommitted the topic branch which contains the Graphviz source code file.

[yaahc]

Awesome, ty Chas. It shows that you have a bunch of extra commits in the history on the topic branch. Can you go ahead and squash the commits back down to just one commit adding the new file? You should be able to delete all of the commits after the first one by running this cmd:

git reset --soft 4817d8c 

Then, if there are any changes that you needed from the extra commits that are left in your working copy you can merge them back into the main commit with:

git add .
git commit --amend

Then you'll need to force push to overwrite the old version of the topic branch with extra commits:

git push --force-with-lease

[Thomasdezeeuw]

@yaahc you can also squash the commits on GitHub, might be easier.