Improve error reporting from restic diff

Instead of a stacktrace, restic diff 111 222 now reports:

	Fatal: no matching ID found for prefix "111"

cmd/restic/cmd_diff.go

@@ -55,9 +55,8 @@ func init() {
 func loadSnapshot(ctx context.Context, repo *repository.Repository, desc string) (*restic.Snapshot, error) {
 	id, err := restic.FindSnapshot(ctx, repo, desc)
 	if err != nil {
-		return nil, err
+		return nil, errors.Fatal(err.Error())
 	}
-
 	return restic.LoadSnapshot(ctx, repo, id)
 }
 

internal/restic/backend_find.go

@@ -2,17 +2,24 @@ package restic
 
 import (
 	"context"
-
-	"github.com/restic/restic/internal/errors"
+	"fmt"
 )
 
-// ErrNoIDPrefixFound is returned by Find() when no ID for the given prefix
-// could be found.
-var ErrNoIDPrefixFound = errors.New("no matching ID found")
+// A MultipleIDMatchesError is returned by Find() when multiple IDs with a
+// given prefix are found.
+type MultipleIDMatchesError struct{ prefix string }
 
-// ErrMultipleIDMatches is returned by Find() when multiple IDs with the given
-// prefix are found.
-var ErrMultipleIDMatches = errors.New("multiple IDs with prefix found")
+func (e *MultipleIDMatchesError) Error() string {
+	return fmt.Sprintf("multiple IDs with prefix %s found", e.prefix)
+}
+
+// A NoIDByPrefixError is returned by Find() when no ID for a given prefix
+// could be found.
+type NoIDByPrefixError struct{ prefix string }
+
+func (e *NoIDByPrefixError) Error() string {
+	return fmt.Sprintf("no matching ID found for prefix %q", e.prefix)
+}
 
 // Find loads the list of all files of type t and searches for names which
 // start with prefix. If none is found, nil and ErrNoIDPrefixFound is returned.
@@ -28,7 +35,7 @@ func Find(ctx context.Context, be Lister, t FileType, prefix string) (string, er
 			if match == "" {
 				match = fi.Name
 			} else {
-				return ErrMultipleIDMatches
+				return &MultipleIDMatchesError{prefix}
 			}
 		}
 
@@ -43,7 +50,7 @@ func Find(ctx context.Context, be Lister, t FileType, prefix string) (string, er
 		return match, nil
 	}
 
-	return "", ErrNoIDPrefixFound
+	return "", &NoIDByPrefixError{prefix}
 }
 
 const minPrefixLength = 8

internal/restic/backend_find_test.go

@@ -2,6 +2,7 @@ package restic
 
 import (
 	"context"
+	"strings"
 	"testing"
 )
 
@@ -48,7 +49,7 @@ func TestFind(t *testing.T) {
 	}
 
 	f, err = Find(context.TODO(), m, SnapshotFile, "NotAPrefix")
-	if err != ErrNoIDPrefixFound {
+	if _, ok := err.(*NoIDByPrefixError); !ok || !strings.Contains(err.Error(), "NotAPrefix") {
 		t.Error("Expected no snapshots to be found.")
 	}
 	if f != "" {
@@ -58,8 +59,8 @@ func TestFind(t *testing.T) {
 	// Try to match with a prefix longer than any ID.
 	extraLengthID := samples[0].String() + "f"
 	f, err = Find(context.TODO(), m, SnapshotFile, extraLengthID)
-	if err != ErrNoIDPrefixFound {
-		t.Error("Expected no snapshots to be matched.")
+	if _, ok := err.(*NoIDByPrefixError); !ok || !strings.Contains(err.Error(), extraLengthID) {
+		t.Errorf("Wrong error %v for no snapshots matched", err)
 	}
 	if f != "" {
 		t.Errorf("Find should not return a match on error.")
@@ -67,8 +68,8 @@ func TestFind(t *testing.T) {
 
 	// Use a prefix that will match the prefix of multiple Ids in `samples`.
 	f, err = Find(context.TODO(), m, SnapshotFile, "20bdc140")
-	if err != ErrMultipleIDMatches {
-		t.Error("Expected multiple snapshots to be matched.")
+	if _, ok := err.(*MultipleIDMatchesError); !ok {
+		t.Errorf("Wrong error %v for multiple snapshots", err)
 	}
 	if f != "" {
 		t.Errorf("Find should not return a match on error.")