Add decryption error codes to base.DecryptionError

These should roughly follow 9732cf5932/MatrixSDK/Crypto/Algorithms/MXDecryptionResult.h (L21-L39)

src/crypto/algorithms/megolm.js

@@ -618,7 +618,10 @@ MegolmDecryption.prototype.decryptEvent = async function(event) {
     if (!content.sender_key || !content.session_id ||
         !content.ciphertext
        ) {
-        throw new base.DecryptionError("Missing fields in input");
+        throw new base.DecryptionError(
+            "MEGOLM_MISSING_FIELDS",
+            "Missing fields in input",
+        );
     }
 
     // we add the event to the pending list *before* we start decryption.
@@ -639,6 +642,7 @@ MegolmDecryption.prototype.decryptEvent = async function(event) {
             this._requestKeysForEvent(event);
         }
         throw new base.DecryptionError(
+            "OLM_UNKNOWN_MESSAGE_INDEX",
             e.toString(), {
                 session: content.sender_key + '|' + content.session_id,
             },
@@ -655,6 +659,7 @@ MegolmDecryption.prototype.decryptEvent = async function(event) {
         // scheduled, so we needn't send out the request here.)
         this._requestKeysForEvent(event);
         throw new base.DecryptionError(
+            "MEGOLM_UNKNOWN_INBOUND_SESSION_ID",
             "The sender's device has not sent us the keys for this message.",
             {
                 session: content.sender_key + '|' + content.session_id,
@@ -673,6 +678,7 @@ MegolmDecryption.prototype.decryptEvent = async function(event) {
     // room, so neither the sender nor a MITM can lie about the room_id).
     if (payload.room_id !== event.getRoomId()) {
         throw new base.DecryptionError(
+            "MEGOLM_BAD_ROOM",
             "Message intended for room " + payload.room_id,
         );
     }