Added errors for services

internal/application/errors/validation_error.go

@@ -0,0 +1,15 @@
+package errors
+
+type ValidationError struct {
+	msg string
+}
+
+func NewValidationError(msg string) *ValidationError {
+	return &ValidationError{
+		msg: msg,
+	}
+}
+
+func (e *ValidationError) Error() string {
+	return "Validation error: " + e.msg
+}