envelope: add doc comments to exported errors

Add Go-style doc comments to all exported error variables in the
envelope package.

Signed-off-by: Christian Stewart <christian@aperture.us>

Author: paralin

envelope/errors.go

@@ -2,14 +2,29 @@ package envelope
 
 import "github.com/pkg/errors"
 
-var (
-	ErrNoGrants            = errors.New("envelope has no grants")
-	ErrNoKeypairs          = errors.New("envelope has no keypairs")
-	ErrInvalidThreshold    = errors.New("invalid threshold configuration")
-	ErrContextMismatch     = errors.New("envelope context does not match expected context")
-	ErrInsufficientShares  = errors.New("insufficient shares to unlock envelope")
-	ErrInvalidShareData    = errors.New("invalid share data")
-	ErrInvalidKeypairIndex = errors.New("keypair index out of range")
-	ErrEmptyPayload        = errors.New("payload is empty")
-	ErrDecryptionFailed    = errors.New("envelope decryption failed")
-)
+// ErrNoGrants is returned when the envelope has no grants.
+var ErrNoGrants = errors.New("envelope has no grants")
+
+// ErrNoKeypairs is returned when the envelope has no keypairs.
+var ErrNoKeypairs = errors.New("envelope has no keypairs")
+
+// ErrInvalidThreshold is returned when the threshold configuration is invalid.
+var ErrInvalidThreshold = errors.New("invalid threshold configuration")
+
+// ErrContextMismatch is returned when the envelope context does not match the expected context.
+var ErrContextMismatch = errors.New("envelope context does not match expected context")
+
+// ErrInsufficientShares is returned when there are not enough shares to unlock the envelope.
+var ErrInsufficientShares = errors.New("insufficient shares to unlock envelope")
+
+// ErrInvalidShareData is returned when share data is malformed or invalid.
+var ErrInvalidShareData = errors.New("invalid share data")
+
+// ErrInvalidKeypairIndex is returned when a keypair index is out of range.
+var ErrInvalidKeypairIndex = errors.New("keypair index out of range")
+
+// ErrEmptyPayload is returned when the payload is empty.
+var ErrEmptyPayload = errors.New("payload is empty")
+
+// ErrDecryptionFailed is returned when envelope decryption fails.
+var ErrDecryptionFailed = errors.New("envelope decryption failed")