rclone: return rclone error instead of canceled context

When rclone fails during the connection setup this currently often
results in a context canceled error. Replace this error with the exit
code from rclone.

internal/backend/rclone/backend.go

@@ -13,6 +13,7 @@ import (
 	"os"
 	"os/exec"
 	"sync"
+	"syscall"
 	"time"
 
 	"github.com/cenkalti/backoff/v4"
@@ -244,7 +245,15 @@ func newBackend(cfg Config, lim limiter.Limiter) (*Backend, error) {
 		// ignore subsequent errors
 		_ = bg()
 		_ = cmd.Process.Kill()
-		return nil, errors.Errorf("error talking HTTP to rclone: %v", err)
+
+		// wait for rclone to exit
+		wg.Wait()
+		// try to return the program exit code if communication with rclone has failed
+		if be.waitResult != nil && (err == context.Canceled || errors.Is(err, io.ErrUnexpectedEOF) || errors.Is(err, syscall.EPIPE)) {
+			err = be.waitResult
+		}
+
+		return nil, fmt.Errorf("error talking HTTP to rclone: %w", err)
 	}
 
 	debug.Log("HTTP status %q returned, moving instance to background", res.Status)

internal/backend/rclone/internal_test.go

@@ -41,3 +41,16 @@ func TestRcloneExit(t *testing.T) {
 		rtest.Assert(t, err != nil, "expected an error")
 	}
 }
+
+// restic should detect rclone startup failures
+func TestRcloneFailedStart(t *testing.T) {
+	cfg := NewConfig()
+	// exits with exit code 1
+	cfg.Program = "false"
+	_, err := Open(cfg, nil)
+	var e *exec.ExitError
+	if !errors.As(err, &e) {
+		// unexpected error
+		rtest.OK(t, err)
+	}
+}