Throw warning when settings migration detects custom `env()` calls

[edalzell]

Spent hours trying to figure out why my site wasn't working, to have Jason figure out I was missing SITE_URL in my .env, which was referenced in sites.php config.

I understand that a warning would have been displayed as part of the migration, but this is something IMO that should be really highlighted as it causes one's site to break and it's nearly impossible to figure out.

Perhaps a big warning section at the bottom highlighting the breaking issues?

[jasonvarga]

We'll just make everything red.

[jesseleite]

I was missing SITE_URL in my .env, which was referenced in sites.php config.

Maybe I can have the settings migrator warn if it detects custom env() calls. That said, the migrator is never going to catch all the custom stuff you did in your v2 apps.

Perhaps a big warning section at the bottom highlighting the breaking issues?

The errors and warnings are already in the output, and the summary line at the bottom clearly points out when there are errors and warnings. That said, I do like your idea in #46. Already started hacking on that one 👍