core: Catch uncaught exceptions in ServerImpl

[anarsultanov]

Fixes #6162 A cleanup regarding #6162

[dapengzhang0]

Ping @ejona86

[ejona86]

The discussion for whether we are going to do this is still ongoing in #6162

[ejona86]

So I took another look at this. I'm really surprised that Java 8 with source=7 has the type inferencing to notice that "no checked exceptions can be thrown" within the block, so even though we throw Throwable it doesn't treat it as Throwable (throw t and throw (Throwable) t behave differently). I knew that was a thing with later Java versions, but I got mixed up and thought it wasn't available to us. I guess the last time I looked at it we were supporting source=6?

Since all the tests pass, this looks like a general cleanup, and so would be good to merge. #6162 is still about whether we'll go out of our way to handle checked-but-not-checked exceptions, but any place Java isn't a PITA when re-throwing and we can have one code path, that would be good.

[ejona86]

Approving from a code-cleanup standpoint.

[dapengzhang0]

LGTM