Platform From Group Fails if Platform Selected is not Set Up

[wxtim]

Describe the bug

Technically this is user/site admin error, but it seems like a simple fix: Currently if you have this config:

[platforms]
  [[platform1]]
    ...

[platform groups]
  [my_group]
    platforms = platform0, platform1

You get an error if the first, undefined platform is selected first, despite the fact that the second platform exists and would work.

This should be handle-able.

Pull requests welcome! This is an Open Source project - please consider contributing a bug fix yourself (please read CONTRIBUTING.md before starting any work though).

[oliver-sanders]

This should be handle-able.

How should we handle this? Silently ignoring the missing platform wouldn't be a good idea.

Perhaps a warning or error?

[wxtim]

Re-reading this I'm not sure that it should be handleable!

If we wanted to we could warn rather than erroring?

Else feel free to close

[dpmatthews]

If possible I think this ought to fail to load in the same way as other invalid settings

[oliver-sanders]

Ok, should be easy, needs adding to an existing validation interface:

https://github.com/cylc/cylc-flow/blob/04437b8d4a610e66befa538b3f4fd3336c1b34b7/cylc/flow/platforms.py#L714-L716