Opportunities for improving new user experience

[teodorlu]

As suggested on Slack,

1. I first naïvely ran it in a folder without a deps.edn file, and spent some time figuring out why it didn't work.
2. Then I reran it in an actual project, only to find out I was making it crash because I hadn't supplied any main args
3. Then I figured I needed to sudo apt install graphviz, because of the dependency on dot.

Suggested fixes:

1. Improved error message
2. Show usage on empty args
3. Note that graphviz must be installed in README

I'm not promising a PR, but feel free to ping me if you haven't heard back in a while :)

Teodor

[sparkofreason]

Along similar lines: it would be nice if navigate --show popped up deps-graph/root.svg in the browser. Right now it doesn't seem to do anything.

[teodorlu]

Hey @sparkofreason,

Does clj -A:vizns single --show do what you want? For me, that pops up a window showing the graph, though in a "java" window, not in a browser.