feat: Check the HMAC during decryption

lib/src/errors.dart

@@ -1,3 +1,9 @@
+/// Triggered during X3DH if the signature if the SPK does verify to the actual SPK.
 class InvalidSignatureException implements Exception {
   String errMsg() => 'The signature of the SPK does not match the provided signature';
 }
+
+/// Triggered by the Double Ratchet if the computet HMAC does not match the attached HMAC.
+class InvalidMessageHMACException implements Exception {
+  String errMsg() => 'The computed HMAC does not match the provided HMAC';
+}