only load eslint config when EXTEND_ESLINT environment variable is specified/ do not swallow eslint config errors

[n1ru4l]

Based on the following discussion that got no response:

#7513 (review)
#7513 (comment)
#7510 (comment)

Summary:

Swallowing the eslint config error is not a good idea

The line eslintConfig = eslintCli.getConfigForFile(paths.appIndexJs); can throw in case the provided eslint config is invalid, e.g. when there is a typo or a plugin dependency is missing. Swallowing that error in case the user explicitly sets the EXTEND_ESLINT environment variable (and therefore expects that his eslint config is used) makes debugging such issues unnecessary cumbersome.

The eslint config should only be loaded in case the user wants to load it

The eslint config is never used in case the EXTEND_ESLINT is not set. Therefore it makes sense to only resolve it's path in case the environment variable is set.

[mrmckeb]

This is a good change, thanks. I'll try to take a look next week (I'm on a business trip now) - unless someone else takes a look before then.

The performance issue would be very small though, as this only runs one time at start.

[n1ru4l]

Hey, @mrmckeb, it would be awesome if you could review this pull request once you got some time for it! I still think that this is a major pain-point for using a custom eslint config.

[ianschmitz]

@n1ru4l do you mind resolving the conflict?

[n1ru4l]

@ianschmitz done 👍

[klaaspieter]

I believe this fixes #7552

[mrmckeb]

This looks great, thanks!