Make error message wording consistent (closes #165)

Error messages in mfer/ and internal/cli/ are lowercase except names and
acronyms, carry no "failed to" or command-name prefix, and each wrap names
only the operation and thing the wrapped error does not already name, so a
stacked message names what failed once. Wraps around errors that already
name their operation and path (os and afero path errors, url.Error, the
builder's path errors, the gpg helpers' own errors) are dropped. gpg's
stderr is appended to a gpg failure, and to the error for a signing key gpg
did not report, only when gpg wrote some. errHTTPStatus reads "unexpected
HTTP status"; both inner-not-set sentinels read "inner message not set".
No sentinel, errors.Is result or exit status changes.

Model: opus-5-5

mfer/serialize.go

@@ -20,11 +20,9 @@ const MAGIC string = "ZNAVSRFG"
 var (
 	// errInnerNotSet is returned by generate when the inner manifest is
 	// missing.
-	errInnerNotSet = errors.New("internal error: pbInner not set")
+	errInnerNotSet = errors.New("inner message not set")
 	// errInternal is returned by generateOuter for the same condition.
-	// The two messages differ, and both are load-bearing for callers that
-	// match on text, so they are kept distinct.
-	errInternal = errors.New("internal error")
+	errInternal = errors.New("inner message not set")
 )
 
 // nanosecondsInt32 converts t's nanosecond component to int32.
@@ -65,14 +63,14 @@ func (m *manifest) generate(ctx context.Context) error {
 
 	dat, err := proto.MarshalOptions{Deterministic: true}.Marshal(m.pbOuter)
 	if err != nil {
-		return fmt.Errorf("serialize: marshal outer: %w", err)
+		return fmt.Errorf("marshal outer message: %w", err)
 	}
 
 	m.output = bytes.NewBufferString(MAGIC)
 
 	_, err = m.output.Write(dat)
 	if err != nil {
-		return fmt.Errorf("serialize: write output: %w", err)
+		return fmt.Errorf("write outer message: %w", err)
 	}
 
 	return nil
@@ -95,7 +93,7 @@ func (m *manifest) generateOuter(ctx context.Context) error {
 
 	innerData, err := proto.MarshalOptions{Deterministic: true}.Marshal(m.pbInner)
 	if err != nil {
-		return fmt.Errorf("serialize: marshal inner: %w", err)
+		return fmt.Errorf("marshal inner message: %w", err)
 	}
 
 	// Compress the inner data
@@ -103,12 +101,12 @@ func (m *manifest) generateOuter(ctx context.Context) error {
 
 	zw, err := zstd.NewWriter(idc, zstd.WithEncoderLevel(zstd.SpeedBestCompression))
 	if err != nil {
-		return fmt.Errorf("serialize: create compressor: %w", err)
+		return fmt.Errorf("create compressor: %w", err)
 	}
 
 	_, err = zw.Write(innerData)
 	if err != nil {
-		return fmt.Errorf("serialize: compress: %w", err)
+		return fmt.Errorf("compress inner message: %w", err)
 	}
 
 	_ = zw.Close()
@@ -120,7 +118,7 @@ func (m *manifest) generateOuter(ctx context.Context) error {
 
 	_, err = h.Write(compressedData)
 	if err != nil {
-		return fmt.Errorf("serialize: hash write: %w", err)
+		return fmt.Errorf("hash inner message: %w", err)
 	}
 
 	sha256Hash := h.Sum(nil)
@@ -149,12 +147,12 @@ func (m *manifest) generateOuter(ctx context.Context) error {
 func (m *manifest) signOuter(ctx context.Context) error {
 	sigString, err := m.signatureString()
 	if err != nil {
-		return fmt.Errorf("failed to generate signature string: %w", err)
+		return fmt.Errorf("build signature string: %w", err)
 	}
 
 	sig, signingKey, err := gpgSign(ctx, []byte(sigString), m.signingOptions.KeyID)
 	if err != nil {
-		return fmt.Errorf("failed to sign manifest: %w", err)
+		return err
 	}
 
 	m.pbOuter.Signature = sig
@@ -163,14 +161,14 @@ func (m *manifest) signOuter(ctx context.Context) error {
 	// fingerprint first.
 	fingerprint, err := gpgGetKeyFingerprint(ctx, GPGKeyID(signingKey))
 	if err != nil {
-		return fmt.Errorf("failed to get key fingerprint: %w", err)
+		return err
 	}
 
 	m.pbOuter.Signer = fingerprint
 
 	pubKey, err := gpgExportPublicKey(ctx, GPGKeyID(fingerprint))
 	if err != nil {
-		return fmt.Errorf("failed to export public key: %w", err)
+		return err
 	}
 
 	m.pbOuter.SigningPubKey = pubKey