Validate Configuration files on launch

[n-aspen]

To help prevent unclear errors as a result of misconfiguration and to make sure errors that do occur are handled safely I need to test all configs at launch and before each backup that calls them.

• [ ] Test core config and default log config

  • [x] missing keys
  • [ ] default entries
  • [ ] invalid API key
  • [ ] unresolvable panel domain
  • [ ] unreachable file
  • [ ] Uncastable datatypes

• [ ] Loop through and test every backup config

  • [ ] default entries
  • [ ] missing keys
  • [ ] unreachable file
  • [ ] Uncastable datatypes
  • [ ] That the backup source exists contains files

• [ ] Store the YAML map while the backup is running to prevent it from being modified mid-backup

If any of these checks fail when the program starts or when a backup is reading the config file it needs to exit and warn the user. The warning levels and methods will be laid out in another issue.

[n-aspen]

Also need to validate that the backup source location exists and contains files.