Emit for key backup failures

Signed-off-by: J. Ryan Stinnett <jryans@gmail.com>

src/client.js

@@ -447,6 +447,7 @@ MatrixClient.prototype.initCrypto = async function() {
     );
 
     this.reEmitter.reEmit(crypto, [
+        "crypto.keyBackupFailed",
         "crypto.roomKeyRequest",
         "crypto.roomKeyRequestCancellation",
         "crypto.warning",

src/crypto/index.js

@@ -1023,6 +1023,7 @@ Crypto.prototype._scheduleKeyBackupSend = async function() {
                     ) {
                         // Backup version has changed or this backup version
                         // has been deleted
+                        this.emit("crypto.keyBackupFailed", err.data.errcode);
                         throw err;
                     }
                 }