Spring Log unresolved exceptions at error level

This PR change the log level of unresolved exceptions from debug to error, in order to avoid the "no exception displayed by default" behaviour that we have currently. Are you ok with this change @rstoyanchev?

Comment From: rstoyanchev

@sdeleuze yes. Might as well also change from debug to error in the various HTTP handler adapters (e.g. ReactorHttpHandlerAdapter), see https://github.com/spring-projects/spring-framework/commit/b2ccdfbc9e112e48f001b0dffa1280e6e7fe7122, which would be invoked only in case the ExceptionHandlingWebHandler is not in use. It would be rare but possible.

Also PR #1187 is for the same request as this.

Comment From: sdeleuze

Merged (https://github.com/spring-projects/spring-framework/commit/324c310cbd3a95da73d36a6f868a538742a10c4f)