When a custom management.port is used, the child context is configured
with an EmbeddedServletContainerFactory bean that has the same class
as the parent context’s EmbeddedServletContainerFactory bean. This
ensures that the child context uses the same type of embedded container
as its parent when there are multiple embedded containers on the
classpath. It also causes a failure when the custom
EmbeddedServletContainerFactory subclass cannot be instantiated, for
example because it’s an anonymous inner-class.

This commit improves the diagnostics so that we fail fast with an
information exception message when we detect that the embedded servlet
container factory bean’s class cannot be instantiated.

Closes gh-6193