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

errors/validation.go

@@ -0,0 +1,15 @@
+package errors
+
+type ValidationError struct {
+    message string
+}
+func NewValidationError(message string) *ValidationError {
+    return &ValidationError{ message: message }
+}
+func (err *ValidationError) Error() string {
+    return err.message
+}
+func IsValidationError(err error) bool {
+    _, ok := err.(*ValidationError)
+    return ok
+}