Added information on custom error handlers.

[dimademi]

I have added some information regarding custom error handling.
I found myself digging through the code to understand how to do it, thought it would be useful in the docs.

[dimademi]

One thing i wanted to add regarding this topic but couldn't find the way to do so --

How can you override the default exception handling for failed steps?
I see WorkflowDefinition.DefaultErrorBehavior but how can i edit it for FluentApi created workflows?

If any of you could let tell me the way to do it ill add it to the PR.

Cheers.

[danielgerlag]

This is really an internal mechanism for handling error types, I suppose you could modify or extend it by registering other handlers in the IoC container, as you can do with any internal workflow-core service... however, I don't think this description really captures that or gives good examples on how to do it. Maybe a better approach would be to write a doc on how to replace internal workflow-core services via IoC registration? We could also add a reference to the new middleware feature on the error handling page?

[dimademi]

That's a good idea, I can open a new doc page for internal services that will, over time, contain all services documentation.
As for the middleware feature documentation - we can paste the detailed explanation you wrote here into the docs