Improve error message when `dot` is not available.

[mdickinson]

For example,

(Pdb) sources[0].export_image()
*** WindowsError: [Error 2] The system cannot find the file specified

is not terribly helpful.

I'm having this error. When does it happen? Some missing dependency? Something to do with the site where the file should be written? Or some incompatibility with windows?

[mdickinson]

@federicoressifei: refcycle expects the dot program from Graphviz to be in the path. So if dot works from your command line, refcycle should be happy. If not, you may need to install Graphviz.

[mdickinson]

Note that if you have Graphviz installed and the dot executable is not in your path, the export_image method has a keyword argument that allows you to specify where the executable is. See the docs for more.