[slash-commands] Standardize Reject/Reply errors

[dustinlacewell]

Create standard error types for slash commands.

At minimum we should have errors that can be thrown from down stream services that when caught by slash command base class automatically get routed as interaction replies.

The error types should have an .ephemeral boolean field to indicate whether the reply should be ephemeral or not.

[dustinlacewell]

Devil's advocate: While it is certainly convenient to simply throw an error from some service in order to propagate a response back to the interaction, it's a bit of a hack.

Ideally, services would throw/return locally relevant errors and data at each level of abstraction with the command finally interpreting behavior and appropriately reporting and negotiating interaction semantics.

Nevertheless, this might be a handy underlying convenience when you just want some simple behavior you intend to improve later