Don't print "context canceled" if user terminated

Without breaking API compatibility, this patch allows us to know whether
a returned `cli/StatusError` was caused by a context cancellation or
not, which we can use to provide a nicer UX and not print the Go
"context canceled" error message if this is the cause.

Signed-off-by: Laura Brehm <laurabrehm@hey.com>

cli/command/container/run.go

@@ -324,6 +324,7 @@ func toStatusError(err error) error {
 
 	if strings.Contains(errMsg, "executable file not found") || strings.Contains(errMsg, "no such file or directory") || strings.Contains(errMsg, "system cannot find the file specified") {
 		return cli.StatusError{
+			Cause:      err,
 			Status:     withHelp(err, "run").Error(),
 			StatusCode: 127,
 		}
@@ -331,12 +332,14 @@ func toStatusError(err error) error {
 
 	if strings.Contains(errMsg, syscall.EACCES.Error()) || strings.Contains(errMsg, syscall.EISDIR.Error()) {
 		return cli.StatusError{
+			Cause:      err,
 			Status:     withHelp(err, "run").Error(),
 			StatusCode: 126,
 		}
 	}
 
 	return cli.StatusError{
+		Cause:      err,
 		Status:     withHelp(err, "run").Error(),
 		StatusCode: 125,
 	}

cli/command/container/run_test.go

@@ -290,6 +290,7 @@ func TestRunPullTermination(t *testing.T) {
 	select {
 	case cmdErr := <-cmdErrC:
 		assert.Equal(t, cmdErr, cli.StatusError{
+			Cause:      context.Canceled,
 			StatusCode: 125,
 			Status:     "docker: context canceled\n\nRun 'docker run --help' for more information",
 		})

cli/error.go

@@ -6,6 +6,7 @@ import (
 
 // StatusError reports an unsuccessful exit by a command.
 type StatusError struct {
+	Cause      error
 	Status     string
 	StatusCode int
 }
@@ -14,8 +15,15 @@ type StatusError struct {
 // it is returned as-is, otherwise it generates a generic error-message
 // based on the StatusCode.
 func (e StatusError) Error() string {
-	if e.Status == "" {
+	if e.Status != "" {
-		return "exit status " + strconv.Itoa(e.StatusCode)
+		return e.Status
 	}
-	return e.Status
+	if e.Cause != nil {
+		return e.Cause.Error()
+	}
+	return "exit status " + strconv.Itoa(e.StatusCode)
+}
+
+func (e StatusError) Unwrap() error {
+	return e.Cause
 }