Add a way to validate the config file

[Eijebong]

Right now there's basically nothing checked on startup and it will panic when vigil starts polling (so basically immediately) if something's wrong.

It'd be nice to have a verification step first so we could include stuff like checking that there's no duplicated id in the config file. (Totally didn't open that because I made this mistake twice already...)

[gbonnefille]

I've just encoutered something similar: I used a wrong URL for probe (tcp://a.b.c.d:1234/) and I only had a panic, invalid replica_url without anything else. At least the incorrect string would be helpful.

[tomlawesome]

+1 haven't even got Vigil to launch properly yet because it panicks straight away