Better error message if `perf` isn't installed

[rickwebiii]

I forgot to install perf on a new cloud instance and got an error message that spawn failed due to file not found. It took me a while to figure out that the error message was due to perf not existing, not my binary.

A small UX improvement would be to print an error if perf itself can't be found :)

[djc]

Sounds good -- would you be able to submit a PR for this?

[rickwebiii]

Yeah, I can probably crank one out this afternoon.