Introduce onErrorContinue to Mono

[smaldini]

Interestingly we might need onErrorContinue on Mono since Flux operations themselves produce Mono (collect, next..).

I was discussing with a user who did use an API (CF Java Client) that internally was operating a flux and returned to the user a collected view. Unfortunately the CF Java Client map operation used was producing rare NPE and the user wanted to filter these anomalies instead of fully retrying or interrupting the flow. Since the user had a mono he did workaround this way:

 monoApi.flux()
             .onErrorContinue(Xxxx.class, xxx)
            .next()