Better dotenv error messages (#4464)
The issue message emitted by `TryDotenvProcessEnv` was pretty bad:
`Execution of TryDotenvProcessEnv::read_all failed`. This error message
comes from #3550, which attaches the calling function as the context for
every failed turbo function.
This PR accomplishes 2 main things:
1. Expose an explicit method determining whether a the
`DotenvEnvProcess`'s prior or current env failed
2. Detects a current env failure and uses the error's root cause for the
issue message.
<img width="961" alt="Screen Shot 2023-04-04 at 7 29 52 PM"
src="https://user-images.githubusercontent.com/112982/229944525-d39dbe87-778a-4421-9bc8-632924cd3782.png">
Fixes WEB-851
Tests: https://github.com/vercel/next.js/pull/47937
