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/client.js

@@ -550,6 +550,9 @@ function _decryptEvent(client, event) {
     try {
         client._crypto.decryptEvent(event);
     } catch (e) {
+        console.warn(
+            `Error decrypting event (id=${event.getId()}): ${e}`,
+        );
         if (!(e instanceof Crypto.DecryptionError)) {
             throw e;
         }