Add an additional error class to expose application and/or temporary errors

[rbino]

Is your feature request related to a problem? Please describe.
Currently we have:

    forbidden: Ash.Error.Forbidden
    invalid: Ash.Error.Invalid
    framework: Ash.Error.Framework
    unknown: Ash.Error.Unknown

but once you start adding your own Application errors using Splode there should be an additional class between invalid and framework, something like temporary or a generic application that can apply to things like:

• I'm querying an external API and it's currently unavailable
• You're being rate limited

invalid seems to express a condition that is true always, and framework expresses something that is internal and where the user has no influence. There should be something that expresses "Maybe try in a bit ¯_(ツ)_/¯"

Describe the solution you'd like
Add another error class that can be used from this kind of errors.

Describe alternatives you've considered
@zachdaniel mentioned "a thing you can do with splice where you can write your own splode with your own error classes, and use it on the error returned" but I don't have more context about this