fmsg/errors: isolate app/error into a separate package

These functions are not in any way specific to the app package.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

internal/fmsg/errors.go

@@ -0,0 +1,72 @@
+package fmsg
+
+import (
+	"fmt"
+	"reflect"
+)
+
+// baseError implements a basic error container
+type baseError struct {
+	Err error
+}
+
+func (e *baseError) Error() string {
+	return e.Err.Error()
+}
+
+func (e *baseError) Unwrap() error {
+	return e.Err
+}
+
+// BaseError implements an error container with a user-facing message
+type BaseError struct {
+	message string
+	baseError
+}
+
+// Message returns a user-facing error message
+func (e *BaseError) Message() string {
+	return e.message
+}
+
+// WrapError wraps an error with a corresponding message.
+func WrapError(err error, a ...any) error {
+	if err == nil {
+		return nil
+	}
+	return wrapError(err, fmt.Sprintln(a...))
+}
+
+// WrapErrorSuffix wraps an error with a corresponding message with err at the end of the message.
+func WrapErrorSuffix(err error, a ...any) error {
+	if err == nil {
+		return nil
+	}
+	return wrapError(err, fmt.Sprintln(append(a, err)...))
+}
+
+// WrapErrorFunc wraps an error with a corresponding message returned by f.
+func WrapErrorFunc(err error, f func(err error) string) error {
+	if err == nil {
+		return nil
+	}
+	return wrapError(err, f(err))
+}
+
+func wrapError(err error, message string) *BaseError {
+	return &BaseError{message, baseError{err}}
+}
+
+var (
+	baseErrorType = reflect.TypeFor[*BaseError]()
+)
+
+func AsBaseError(err error, target **BaseError) bool {
+	v := reflect.ValueOf(err)
+	if !v.CanConvert(baseErrorType) {
+		return false
+	}
+
+	*target = v.Convert(baseErrorType).Interface().(*BaseError)
+	return true
+}

internal/fmsg/fmsg.go

@@ -0,0 +1,2 @@
+// Package fmsg provides various functions for output messages.
+package fmsg