RFC - error_chain in some examples is quite noisy

[budziq]

@dtolnay Hi after switching all examples with fn run() to error_chain some of the examples became quite noisy eg http://localhost:3000/net.html#parse-a-url-from-a-string-to-a-url-type
has 8 lines of code to 9 lines of error_chain maiking it hard to read

I think that we could have best of both worlds (completely copy-pastable examples and minimal noise) if we have used the expand collapse button (currently hidden along with play button) and hide the error_chain and main boiler plate by default (and be able to show it on click).

I will prepare a minimal PR for your consideration