errors: Ensure that errors.IsFatal(errors.Fatal("err")) == true

This fixes a few cases where restic output "Fatal: Fatal: [...]"

internal/errors/fatal_test.go

@@ -0,0 +1,22 @@
+package errors_test
+
+import (
+	"testing"
+
+	"github.com/restic/restic/internal/errors"
+)
+
+func TestFatal(t *testing.T) {
+	for _, v := range []struct {
+		err      error
+		expected bool
+	}{
+		{errors.Fatal("broken"), true},
+		{errors.Fatalf("broken %d", 42), true},
+		{errors.New("error"), false},
+	} {
+		if errors.IsFatal(v.err) != v.expected {
+			t.Fatalf("IsFatal for %q, expected: %v, got: %v", v.err, v.expected, errors.IsFatal(v.err))
+		}
+	}
+}