Modify run_with_result() to return Result<(), Box<dyn std::error::Error>>

[timburks]

Addresses #78

This eliminates the ActionResult type and moves ActionError into the error.rs file alongside FlagError. To allow it to be returned in Box<> values, ActionError is a struct with an associated enum ActionErrorKind. This is different from FlagError, which is an enum, and for consistency we may want to rename FlagError to FlagErrorKind and add a new FlagError struct that is similar to ActionError.

This is a breaking change from v2.2.0 but only in the types returned by run_with_result and the functions of type ActionWithResult. I think it is worth making this change for consistency with general Rust best practices, as described in Recoverable Errors with Result.

[ksk001100]

@timburks
LGTM

[timburks]

@ksk001100 Thanks for merging! For future PRs, you might want to choose the "squash and merge" option to combine all of the commits in the PR into a single commit. I haven't been trying to minimize the number of commits in my PRs and don't want my work-in-progress commits to make your commit history ugly! :)

[ksk001100]

@timburks
Thank you for sharing the "squash and merge" option. I did not know this feature existed.