Show nicer error when a @ConfigMapping class exists in the deployment module and set to the runtime phase

[gastaldi]

Description

While creating (see this PR for more context - quarkusio/quarkus-workshops#600), I noticed that the following error is thrown when the @ConfigMapping phase is set to RUN_TIME:

Caused by: java.lang.ClassNotFoundException: io.quarkus.workshop.superheroes.version.deployment.VersionConfig
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:525)
	at io.quarkus.bootstrap.classloading.QuarkusClassLoader.loadClass(QuarkusClassLoader.java:533)
	at io.quarkus.bootstrap.classloading.QuarkusClassLoader.loadClass(QuarkusClassLoader.java:481)
	at io.quarkus.bootstrap.classloading.QuarkusClassLoader.loadClass(QuarkusClassLoader.java:533)
	at io.quarkus.bootstrap.classloading.QuarkusClassLoader.loadClass(QuarkusClassLoader.java:481)
	at io.quarkus.runtime.configuration.AbstractConfigBuilder.withMapping(AbstractConfigBuilder.java:64)
	... 21 more

The solution was to move the @ConfigMapping to the runtime module, but the error message is a bit cryptic.

• Zulip thread: https://quarkusio.zulipchat.com/#narrow/stream/187030-users/topic/Workshop/near/471292983

Implementation ideas

It would be nice if we could detect that somehow and display a nicer error message

[quarkus-bot]

/cc @radcortez (config)

[quarkus-bot]

You added a link to a Zulip discussion, please make sure the description of the issue is comprehensive and doesn't require accessing Zulip

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[mcruzdev]

I would like to contribute with this one @gastaldi!

[radcortez]

Feel free to look into it.

This also applies to old @ConfigRoot class objects.