Print help text on specific domain exceptions

[domn1995]

Closes #16.

Notable Changes

• Added showHelp: true parameter to end-user-facing CliFxExceptions.
• Modified BaseCliFxException to contain a virtual ExitCode property that returns its HResult by default. CommandException's just override this property to set the exit code passed in its constructor or to the default exit code of -100. This greatly simplified exception handling in CliApplication and is a net reduction of 18 lines of code :)

@Tyrrrz, can you double check that you're happy with these changes? If you like it, how would you prefer I implement unit tests? Should I augment the existing ArgumentBindingSpecs that throw the end-user-facing exceptions to also check that we're printing the help text or make brand new ones that assert we're throwing a CliFxException AND displaying the help text?

[Tyrrrz]

I'm not a big fan of virtual properties. I think it's better to have it as an auto-property and set it through the base constructor (it can default to HResult in one of the overloads).

Regarding unit tests: we don't write any :) As for functional tests, I'd say add a test to ErrorReportingSpecs any of the simpler user-facing errors to make sure it also contains help text.

[domn1995]

@Tyrrrz, ready for your input again. I've gone ahead and moved the exit code logic to BaseCliFxException and added an FT for showing the help text on invalid user input.

[Tyrrrz]

That looks good. Now I'm wondering, should we just merge BaseCliFxException and CliFxException (and have CommandException inherit from CliFxException)? What do you think? I don't know if there's much benefit in keeping them separate.

[domn1995]

Yeah, I think that's a good idea. Want me to update the PR?

[Tyrrrz]

Sure, go ahead.

[domn1995]

@Tyrrrz, done and hopefully ready for a merge :)

[Tyrrrz]

Thanks!