Env Var validation in generated scripts

[dylan-smith]

The migration scripts produced by generate-script expect certain environment variables to be set in order to work. To improve the customer experience we could have a bit of pwsh at the start of the script that validates the appropriate env vars are set and give the user a useful error message if not.

[hfishback01]

I believe this is already addressed here. Shall I close? @dylan-smith @ArinGhazarian

[timrogers]

@hfishback01 That's a good spot! I think the issue of this issue was to add a more explicit validation error to the top of the script to make it a bit clearer - it could say, for example, "you must set environment variables X, Y and Z when running a migration script". Maybe it isn't strictly necessary, but I think it'll help users.