SpringBoot Polish SpringApplication to avoid dead code after calls to handleRunFailure

As reported in #38951, there's some dead code after calls to handleRunFailure. We think it might be better to have handleRunFailure return the exception to be thrown and then have the caller throw it.

Comment From: BenchmarkingBuffalo

Hi @wilkinsona, I would like to work on this issue. If you want, you can assign it to me and I will open a PR for it.

Comment From: philwebb

Closing in favor of PR #39107. Thanks @BenchmarkingBuffalo!