[Bug] Error message on missing checkout action is sub-optimal

[Vampire]

Component

• github-workflows-kt (library with the DSL)
• bindings server (https://bindings.krzeminski.it)
• I don't know

Action

n/a

Expected

Better and actionable error message or fallback version

Actual

Since 4.0.0 inferring the checkout action version from classpath is the default.
If you have a workflow without checkout action, the error message does not make too much sense anymore.
It states

java.lang.IllegalStateException: actions/checkout is not found in the classpath! Either add a dependency on it (`@file:DependsOn("actions:checkout:<version>")`), or don't use CheckoutActionVersionSource.InferFromClasspath()

This was fine when InferFromClasspath was used explicitly.
But with it being used implicitly saying "don't use it" is not really applicable and not actionable for someone not knowing where it needs to be changed.

It would be better to tell the user what to use and how, like with the DependsOn part,
or to use a default version if none is found in the classpath which should anyway be a rare case.
(rare but it exists: https://github.com/Vampire/setup-wsl/blob/master/.github/workflows/verify-release.main.kts)

Workaround, if exists

n/a

Version

4.0.0

[krzema12]

@Vampire I recommend this kind of change - WDYT?

 java.lang.IllegalStateException: actions/checkout is not found in the classpath!
-Either add a dependency on it (`@file:DependsOn("actions:checkout:<version>")`),
-or don't use CheckoutActionVersionSource.InferFromClasspath()
+Most often, adding a dependency on it (`@file:DependsOn("actions:checkout:<version>")`)
+is what gives best maintainability (all versions defined in a single place). It's also possible
+to opt out from the default CheckoutActionVersionSource.InferFromClasspath() as
+workflow.consistencyCheckJobConfig.checkoutActionVersion, and use CheckoutActionVersionSource
+that gives more control.

[krzema12]

Hmm, we could also mention the checkoutless workflows, too.

[Vampire]

I think this is only about checkoutless workflows, isn't it?
Hence the "all versions defined in a single place" is not really correct.
If it is not a checkoutless workflow it would have been found in the classpath.

[krzema12]

@Vampire ok, see #2390 - it's a very simple adjustment that keeps the message simple, but should do the job.