Add SendError struct implementing the error interface

Fixes #12

[Ivy:
 - squash commits and revise commit message
 - tweak SendError to preserve both zero-index and original error message
 - improve documentation of SendError struct
 - add changelog entry]

Author: beeceej

CHANGELOG.md

@@ -2,6 +2,13 @@
 All notable changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/).
 
+## [2.3.0] - *Unreleased*
+
+### Added
+
+- #12: Adds `SendError` to provide additional info about the cause and index of
+  a failed attempt to transmit a batch of messages.
+
 ## [2.2.0] - 2018-03-01
 
 ### Added

errors.go

@@ -0,0 +1,16 @@
+package mail
+
+import "fmt"
+
+// A SendError represents the failure to transmit a Message, detailing the cause
+// of the failure and index of the Message within a batch.
+type SendError struct {
+	// Index specifies the index of the Message within a batch.
+	Index uint
+	Cause error
+}
+
+func (err *SendError) Error() string {
+	return fmt.Sprintf("gomail: could not send email %d: %v",
+		err.Index+1, err.Cause)
+}

send.go

@@ -36,7 +36,7 @@ func (f SendFunc) Send(from string, to []string, msg io.WriterTo) error {
 func Send(s Sender, msg ...*Message) error {
 	for i, m := range msg {
 		if err := send(s, m); err != nil {
-			return fmt.Errorf("gomail: could not send email %d: %v", i+1, err)
+			return &SendError{Cause: err, Index: uint(i)}
 		}
 	}