HEAVY: finish music service migration, tidy up services, more tests

errors/notexist.go

@@ -0,0 +1,16 @@
+package errors
+
+type NotExistError struct {
+    query string
+}
+func NewNotExistError(query string) *NotExistError {
+    return &NotExistError{ query: query }
+}
+func (err *NotExistError) Error() string {
+    return err.query
+}
+func IsNotExistError(err error) bool {
+    _, ok := err.(*NotExistError)
+    return ok
+}
+