Clean up/improve e2e logging

In an attempt to make the rageshake logs a bit more useful, try to make the
logging a bit saner. Firstly, make sure we log every decryption failure, and
log it exactly once, rather than in several places. Also record when we receive
megolm keys. Also add some more explicit logging in the sync loop.

src/crypto/algorithms/megolm.js

@@ -544,7 +544,11 @@ MegolmDecryption.prototype.decryptEvent = function(event) {
         if (e.message === 'OLM.UNKNOWN_MESSAGE_INDEX') {
             this._addEventToPendingList(event);
         }
-        throw new base.DecryptionError(e);
+        throw new base.DecryptionError(
+            e.toString(), {
+                session: content.sender_key + '|' + content.session_id,
+            },
+        );
     }
 
     if (res === null) {
@@ -552,6 +556,9 @@ MegolmDecryption.prototype.decryptEvent = function(event) {
         this._addEventToPendingList(event);
         throw new base.DecryptionError(
             "The sender's device has not sent us the keys for this message.",
+            {
+                session: content.sender_key + '|' + content.session_id,
+            },
         );
     }
 
@@ -593,26 +600,27 @@ MegolmDecryption.prototype._addEventToPendingList = function(event) {
  * @param {module:models/event.MatrixEvent} event key event
  */
 MegolmDecryption.prototype.onRoomKeyEvent = function(event) {
-    console.log("Adding key from ", event);
     const content = event.getContent();
     const senderKey = event.getSenderKey();
+    const sessionId = content.session_id;
 
     if (!content.room_id ||
-        !content.session_id ||
+        !sessionId ||
         !content.session_key ||
         !senderKey
        ) {
-        console.error("key event is missing fields");
+        console.error(`key event is missing fields`);
         return;
     }
 
+    console.log(`Adding key for megolm session ${senderKey}|${sessionId}`);
     this._olmDevice.addInboundGroupSession(
-        content.room_id, senderKey, content.session_id,
+        content.room_id, senderKey, sessionId,
         content.session_key, event.getKeysClaimed(),
     );
 
     // have another go at decrypting events sent with this session.
-    this._retryDecryption(senderKey, content.session_id);
+    this._retryDecryption(senderKey, sessionId);
 };