Spring Optimize uses of onErrorResume()

Replace uses of onErrorResume() with - onErrorMap() in places where it is just used to map to a different exception. - onErrorComplete() where it just maps to Mono.empty(). - onErrorReturn() where it just maps to Mono.just().

The first change achieves the same using less code, while the latter two changes improve efficiency as of Reactor 3.4.23 (see reactor/reactor-core#3159) by avoiding the unnecessary creation of Mono/Flux instances.

Comment From: bclozel

Thanks @kzander91 !