Show friendlier error in `am start` when scrape target is not yet up

[brettimus]

when you am start :1337 but port 1337 has no /metrics route, then you get a warning log in the console. it'd be nice to say something like "target not yet up, but prometheus will still try to scrape it" (don't use that exact copy pls)

[hatchan]

This is running in verbose mode, when running in "normal" mode it is more clean though.