Now that CascadiaSettings is a WinRT object, we need to update the error
handling a bit. Making it a WinRT object limits our errors to be
hresults. So we moved all the error handling down a layer to when we
load the settings object.
- Warnings encountered during validation are saved to `Warnings()`.
- Errors encountered during validation are saved to `GetLoadingError()`.
- Deserialization errors (mainly from JsonUtils) are saved to
`GetDeserializationErrorMessage()`.
## References
#7141 - CascadiaSettings is a settings object
#885 - this makes ripping out CascadiaSettings into
TerminalSettingsModel much easier
## Validation Steps Performed
* [x] Tests passed
- [x] Deployment succeeded
- tested with invalid JSON (deserialization error)
- tested with missing DefaultProfile (validation error)
