Avoid calling exit from inside library, and removed "create-genesis-json" startup option

[jmjatlanta]

Fixes #1110

Calling exit from inside a library makes unit testing impossible. This fix uses return values to indicate if the application should end or continue.

[pmconrad]

IMO the --create-example-genesis option does not serve a useful purpose.
Things could be simplified by creating an example genesis file somewhere in the documentation, removing that option from the code, and throwing in case of an error instead of defining and checking return codes. Return codes are error-prone, which is why exceptions were invented.

[jmjatlanta]

IMO the --create-example-genesis option does not serve a useful purpose.

I can certainly do that. I do believe that option is part of some instructions (private testnet perhaps?), so cleaning the documentation would be required. @abitmore I would appreciate your opinion.

[abitmore]

I prefer keeping the code if it leads to less work.

[pmconrad]

Accepting this PR will be the least work of course, but IMO the end result would be better if my suggestion was implemented.

[pmconrad]

Please resolve conflicts, then I'll approve.

[abitmore]

Created issue #1536 for tracking documentation work.

[pmconrad]

Thanks!