Add secret.Warn() calls for all silent anomalous conditions

Audit of the codebase found 9 locations where errors or anomalous
conditions were silently swallowed or only logged via Debug(). Users
should be informed when something unexpected happens, even if the
program can continue.

Changes:
- DetermineStateDir: warn on config dir fallback to ~/.config
- info_helper: warn when vault/secret stats cannot be read
- unlockers list: warn on metadata read/parse failures (fixes FIXMEs)
- unlockers list: warn on fallback ID generation
- checkUnlockerExists: warn on errors during duplicate checking
- completions: warn on unlocker metadata read/parse failures
- version list: upgrade metadata load failure from Debug to Warn
- secrets: upgrade file close failure from Debug to Warn
- version naming: warn on malformed version directory names

Closes #19

internal/cli/secrets.go

@@ -507,7 +507,7 @@ func (cli *Instance) ImportSecret(cmd *cobra.Command, secretName, sourceFile str
 	}
 	defer func() {
 		if err := file.Close(); err != nil {
-			secret.Debug("Failed to close file", "error", err)
+			secret.Warn("Failed to close file", "error", err)
 		}
 	}()