Polish
Drop getPort from ConfEmbServletContainerFactory

Remove getPort() from ConfigurableEmbeddedServletContainerFactory to
keep the interface primarily setters.
Tweak ConfigurableEmbeddedServletContainerFactory

Update ConfigurableEmbeddedServletContainerFactory to no longer directly
extend EmbeddedServletContainerFactory.
Tidy ConfigurableEmbeddedServletContainerFactory

Rename ConfigurableEmbeddedServletContainerFactory to
ConfigurableEmbeddedServletContainer and extract
AbstractConfigurableEmbeddedServletContainer from
AbstractEmbeddedServletContainerFactory.
Rework ErrorWrapperEmbeddedServletContainerFactory

Rename ErrorWrapperEmbeddedServletContainerFactory to ErrorPageFilter
and extend AbstractConfigurableEmbeddedServletContainer rather
than AbstractEmbeddedServletContainerFactory.

Fixes gh-416