Added errors for services

internal/application/errors/already_exists_error.go

@@ -0,0 +1,15 @@
+package errors
+
+type AlreadyExistsError struct {
+	msg string
+}
+
+func NewAlreadyExistsError(msg string) *AlreadyExistsError {
+	return &AlreadyExistsError{
+		msg: msg,
+	}
+}
+
+func (e *AlreadyExistsError) Error() string {
+	return "Already exists: " + e.msg
+}