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/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 == "" {
-		return "exit status " + strconv.Itoa(e.StatusCode)
+	if e.Status != "" {
+		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
 }