Add warning when GraphViz is not installed

[clue]

Currently, generating a graph image just fails if GraphViz (dot executable) is not installed. Instead, we should check for the existence and fail gracefully with a concise, readable error message. On Debian-based systems, we could perhaps also offer to install it via sudo apt-get install graphviz.

[fzaninotto]

Agreed, the current error message:

Unable to invoke "dot" to create image file  (code 127)

Doesn't speak to those who don't know that dot is GraphViz. You should also update the installation manual to explain that downloading the phar is not enough.

[clue]

You should also update the installation manual

Indeed, thanks for commenting! As a first step, I've just updated the install instructions (d322855).

Anyway, I'll leave this open until we fail gracefully with a concise, readable error message.