Support customization of the failure messages coming from `xcode_config`

This is something I've wanted for ages but couldn't do while the rules were defined in bazel-core. Regarding this message:

> Error in fail: --xcode_version=17C529 specified, but '17C529' is not an available Xcode version. If you believe you have '17C529' installed, try running "bazel shutdown", and then re-run your command.

I'd like to be able to steer my developers towards our internal documentation for installing Xcode, or provide guidance towards our support channels, directly from the failure messages produced by the rule. I'm aware I could do this in my `tools/bazel` wrapper, and maybe a better outcome from this ticket would be a less obtuse message. But the wording of this particular message is a common source of confusion on my team, so I'd like to make this more extensible. Thoughts?

Start by locating the xcode_config rule and the code path that emits the shown failure message. Read how its failure text is currently constructed and check existing tests around Xcode version validation. Done means providing a documented customization point for the message while preserving the existing diagnostic behavior.

Written by the indexing model from the issue text.