Add `ConditionsNotMet` error

Add a `ConditionsNotMet` error that is for endpoints where the action cannot be completed because of some non-argument, non-auth related condition or requirement. E.g., no files specified and failed to find files in the default location.

The issue does not name a file or test; first inspect the repository's existing error definitions and endpoint-related conventions. Confirm how non-argument, non-auth conditions are represented, then verify that the new error covers cases such as missing files with no default-location matches.

Written by the indexing model from the issue text.