Yarn won't change offline-cache folder if the current offline-cache folder isn't permitted to create

[paleite]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
Yarn doesn't allow me to change the folder for yarn-offline-mirror, if it doesn't have write access to the current one.

If the current behavior is a bug, please provide the steps to reproduce.
After running yarn config set yarn-offline-mirror ../npm-packages-offline-cache (not the two dots in the path) it saves the config, but doesn't try to create the folder. After that, I tried to change the folder, but yarn throws an error saying

error An unexpected error occurred: "EACCES: permission denied, mkdir '/Users/npm-packages-offline-cache'".

i.e. it tries to create that dir, realises it can't then fails to update the config.

What is the expected behavior?
To be able to change the folder for yarn-offline-mirror, even if it doesn't have write access to the current one.

I expect 2 parts:

1. When setting the folder for yarn-offline-mirror, try to create the folder.
   • If it fails, don't write to the config and throw error.
2. When trying to change offline-mirror config, first allow the config to be changed, then try to create the folder.

Please mention your node.js, yarn and operating system version.
node v10.6.0
yarn 1.7.0
osx 10.13.4