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/secret/version.go

@@ -102,6 +102,8 @@ func GenerateVersionName(fs afero.Fs, secretDir string) (string, error) {
 
 		var serial int
 		if _, err := fmt.Sscanf(parts[1], "%03d", &serial); err != nil {
+			Warn("Skipping malformed version directory name", "name", entry.Name(), "error", err)
+
 			continue
 		}