[bugfix] internal server error on search not found (#1590)

* add error value wrapping, include status code / not found flags from transport errors, update error usages

Signed-off-by: kim <grufwub@gmail.com>

* add code commenting for gtserror functions

Signed-off-by: kim <grufwub@gmail.com>

---------

Signed-off-by: kim <grufwub@gmail.com>

internal/gtserror/error.go

@@ -0,0 +1,60 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2023 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package gtserror
+
+import (
+	"codeberg.org/gruf/go-errors/v2"
+)
+
+// package private error key type.
+type errkey int
+
+const (
+	// error value keys.
+	_ errkey = iota
+	statusCodeKey
+	notFoundKey
+)
+
+// StatusCode checks error for a stored status code value. For example
+// an error from an outgoing HTTP request may be stored, or an API handler
+// expected response status code may be stored.
+func StatusCode(err error) int {
+	i, _ := errors.Value(err, statusCodeKey).(int)
+	return i
+}
+
+// WithStatusCode will wrap the given error to store provided status code,
+// returning wrapped error. See StatusCode() for example use-cases.
+func WithStatusCode(err error, code int) error {
+	return errors.WithValue(err, statusCodeKey, code)
+}
+
+// NotFound checks error for a stored "not found" flag. For example
+// an error from an outgoing HTTP request due to DNS lookup.
+func NotFound(err error) bool {
+	_, ok := errors.Value(err, notFoundKey).(struct{})
+	return ok
+}
+
+// SetNotFound will wrap the given error to store a "not found" flag,
+// returning wrapped error. See NotFound() for example use-cases.
+func SetNotFound(err error) error {
+	return errors.WithValue(err, notFoundKey, struct{}{})
+}