The handler should always return an integer

[adamsitnik]

Makes progress towards #2071 and meeting BCL requirements

[adamsitnik]

@jonsequitur Based on our offline discussion yesterday I know that:

• forcing the users to always return an exit code from handler might be verbose, but on the other hand there is no magic (so far invoke/invokeasync were returning integer, while handlers were returning void) which aligns well with the BCL guidelines
• the removal of InvocationResult should be seen as "we don't have a good proposal now, so we are removing it until we have one" rather than "everything will always return an int"

Since I want to introduce CliAction today and replace ICommandHandler I am going to merge this PR right now to avoid massive conflict.