Improve decryption error messages

Attempt to make the decryption errors less obscure

lib/client.js

@@ -467,7 +467,7 @@ MatrixClient.prototype.isRoomEncrypted = function(roomId) {
  */
 function _decryptEvent(client, event) {
     if (!client._crypto) {
-        _badEncryptedMessage(event, "**Encryption not enabled**");
+        _badEncryptedMessage(event, "Encryption not enabled");
         return;
     }
 
@@ -477,7 +477,7 @@ function _decryptEvent(client, event) {
         if (!(e instanceof Crypto.DecryptionError)) {
             throw e;
         }
-        _badEncryptedMessage(event, "**" + e.message + "**");
+        _badEncryptedMessage(event, e.message);
         return;
     }
 }
@@ -487,7 +487,7 @@ function _badEncryptedMessage(event, reason) {
         type: "m.room.message",
         content: {
             msgtype: "m.bad.encrypted",
-            body: reason,
+            body: "** Unable to decrypt: " + reason + " **",
         },
     });
 }

lib/crypto/algorithms/megolm.js

@@ -458,7 +458,9 @@ MegolmDecryption.prototype.decryptEvent = function(event) {
     if (res === null) {
         // We've got a message for a session we don't have.
         this._addEventToPendingList(event);
-        throw new base.DecryptionError("Unknown inbound session id");
+        throw new base.DecryptionError(
+            "The sender's device has not sent us the keys for this message."
+        );
     }
 
     var payload = JSON.parse(res.result);

lib/crypto/index.js

@@ -1220,7 +1220,9 @@ Crypto.prototype._getRoomDecryptor = function(roomId, algorithm) {
 
     var AlgClass = algorithms.DECRYPTION_CLASSES[algorithm];
     if (!AlgClass) {
-        throw new algorithms.DecryptionError("Unable to decrypt " + algorithm);
+        throw new algorithms.DecryptionError(
+            'Unknown encryption algorithm "' + algorithm + '".'
+        );
     }
     alg = new AlgClass({
         userId: this._userId,