warn and exit when `xcrun` is not installed

[swyxio]

Environment

Description

i have cocoapods installed

but the cli thinks i dont

i think this is because i have an implicit dependency on xcode and i didnt know i needed it

we should detect this first and prompt

[swyxio]

furthermore, i've been using xcode 11 beta, and it looks like i need xcode 10 so i am downgrading.

[swyxio]

ok it looks like my upgrading os to Mojave borked git which borked cocoapods which produced this very unintuitive error. chatted with Clint Rutkas about maybe setting up a brew script to verify and install all this..

[thymikee]

We can print the error output when it's failing. Would that help?

[swyxio]

yes! would be better than what i suggested.

[grabbou]

We should also add this to doctor.