feat(errors): name the command a timeout was waiting on

[route]

Refs #470.

The problem

Ferrum::TimeoutError said only that a response never came:

Timed out waiting for response. It's possible that this happened because something took a very long time...

Which command, on which session, isn't in there. #470 is the cost of that: it took a year, three reporters and a full FERRUM_DEBUG transcript to establish that the command left unanswered was Page.enable on a freshly created page.

The change

The command and the session are part of the message, and readable off the error:

Timed out waiting for a response to Page.enable (session 93FF304962AB15CD3FF17CEF057BB1B8). It's possible that...

rescue Ferrum::TimeoutError => e
  e.command     # "Page.enable"
  e.session_id  # "93FF304962AB15CD3FF17CEF057BB1B8"
end

Only Client#send_message passes them, since it's the only place that waits on a specific command. The waits that aren't for one, Network#wait_for_idle and Page#command's post-command event wait, raise the same message as before, and a browser-session command gets a response to Target.createTarget with no session part.

Verification

• New example in spec/client_spec.rb: a Page.navigate to the app's slow route with a short timeout raises with a response to Page.navigate (session ...).
• Full suite: 616 examples, 0 failures.