Support exiting if configuration is locked by another process

Perhaps print a message and exit.

I am developing tools for Python projects that depend on Julia. I search for Julia installed by juliaup. Detecting if `juliaup` is not returning because the config file is locked is a bit of a trick. It's not Python-specific of course. Other projects that call `juliaup` will have to roll their own solution.

This is similar to #253. I good solution to these things might be an API for juliaup that uses machine readable input and output. Maybe json in and out.

Trace juliaup's configuration access and its behavior when the config file is locked, then read the related issue #253 for context. Define the user-facing message and exit behavior, and verify the result through juliaup's existing invocation.

Written by the indexing model from the issue text.