NotesService: Throw NotInDBError when the note wasn't found

Signed-off-by: David Mehren <git@herrmehren.de>

src/errors/errors.ts

@@ -0,0 +1,7 @@
+export class NotInDBError extends Error {
+  name = 'NotInDBError';
+}
+
+export class ClientError extends Error {
+  name = 'ClientError';
+}