Throw `ContextLoadException` from `SpringBootContextLoader`

[sbrannen]

#31793 introduced a custom ApplicationContextFailureProcessor; however, that is half of the support needed for processing ApplicationContext load failures.

The other half is the new requirement that a SmartContextLoader catch any exception thrown in loadContext() and throw it wrapped in a ContextLoadException.

See #31793 (comment) for details.

[sbrannen]

@scottfrederick, do you think you can squeeze this in before 3.0 RC2?

[scottfrederick]

Since #31793 hasn't shipped yet, we'll re-open that issue and address this last bit there.