chore(all): review error wrappings

- remove repetitive `cannot` and `failed` prefixes
- rename `unmarshaling` to `decoding`

internal/configuration/settings/dns.go

@@ -31,7 +31,7 @@ type DNS struct {
 func (d DNS) validate() (err error) {
 	err = d.DoT.validate()
 	if err != nil {
-		return fmt.Errorf("failed validating DoT settings: %w", err)
+		return fmt.Errorf("validating DoT settings: %w", err)
 	}
 
 	return nil

internal/configuration/settings/openvpn.go

@@ -171,7 +171,7 @@ func validateOpenVPNConfigFilepath(isCustom bool,
 	extractor := extract.New()
 	_, _, err = extractor.Data(confFile)
 	if err != nil {
-		return fmt.Errorf("failed extracting information from custom configuration file: %w", err)
+		return fmt.Errorf("extracting information from custom configuration file: %w", err)
 	}
 
 	return nil