Feature: don't fail the build if the site doesn't match the requirements

[erezrokah]

The plugin currently fails the build if:

1. This site is not a next site
2. The site is a static HTML export Next.js site
3. The site has next-on-netlify installed
4. The site configures a target other than serverless or experimental-serverless-trace

Per https://github.com/netlify/pod-workflow/issues/121#issuecomment-763411132 I suggest to change it to only fail if it's not a next site.

For all other cases we can print a relevant message and do nothing.

[lindsaylevine]

thanks for opening this officially!! this has been on the dx internal board/on my radar for several weeks but it's good to have here as well!!! :)