Adding client enumeration mitigations (#153)

src/errors.rs

@@ -92,6 +92,8 @@ pub enum ProtocolError {
     ///
     /// Internal error during password verification: {0}
     VerificationError(PakeError),
+    /// This error occurs when the inner envelope is malformed
+    InvalidInnerEnvelopeError,
     /// This error occurs when the server answer cannot be handled
     /// Server response cannot be handled.
     ServerError,