Flatten underlying errors

[dangthaison91]

This will resolve #119.

After referring Action's implementation from ReactiveSwift, I decide to add disabledErrors: Observable<Error> and remove ActionError. This will make breaking change but will help end users avoid misunderstand between .errors/.underlyingErrors and .executionErrors from the original proposal. I add ActionDisabledError for better debugging.

What do you think @bobgodwinx

[rxswiftcommunity]

	Warnings
⚠️	It looks like code was changed without adding anything to the Changelog. If this is a trivial PR that doesn't need a changelog, add #trivial to the PR title or body.

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[mosamer]

@dangthaison91 as this is a breaking change, would you mind to include it as part of release 4.0, and point this PR to 4.0branch instead of master?

[dangthaison91]

Thanks for prompting me @mosamer. Changed to branch 4.0.