queue keys for backup even if backup isn't enabled yet

We may not have managed to set up the backup yet when we get keys.  So we should
unconditionally queue up the keys for backup, so that when the backup is set up,
they will be sent instead of dropped.

src/crypto/algorithms/megolm.js

@@ -369,17 +369,11 @@ MegolmEncryption.prototype._prepareNewSession = async function() {
         key.key, {ed25519: this._olmDevice.deviceEd25519Key},
     );
 
-    if (this._crypto.backupInfo) {
     // don't wait for it to complete
     this._crypto.backupGroupSession(
         this._roomId, this._olmDevice.deviceCurve25519Key, [],
         sessionId, key.key,
-        ).catch((e) => {
-            // This throws if the upload failed, but this is fine
-            // since it will have written it to the db and will retry.
-            logger.log("Failed to back up megolm session", e);
-        });
-    }
+    );
 
     return new OutboundSessionInfo(sessionId);
 };
@@ -1347,18 +1341,12 @@ MegolmDecryption.prototype.onRoomKeyEvent = function(event) {
                 }
             });
     }).then(() => {
-        if (this._crypto.backupInfo) {
         // don't wait for the keys to be backed up for the server
         this._crypto.backupGroupSession(
             content.room_id, senderKey, forwardingKeyChain,
             content.session_id, content.session_key, keysClaimed,
             exportFormat,
-            ).catch((e) => {
-                // This throws if the upload failed, but this is fine
-                // since it will have written it to the db and will retry.
-                logger.log("Failed to back up megolm session", e);
-            });
-        }
+        );
     }).catch((e) => {
         logger.error(`Error handling m.room_key_event: ${e}`);
     });
@@ -1564,7 +1552,7 @@ MegolmDecryption.prototype.importRoomKey = function(session, opts = {}) {
         true,
         opts.untrusted ? { untrusted: opts.untrusted } : {},
     ).then(() => {
-        if (this._crypto.backupInfo && opts.source !== "backup") {
+        if (opts.source !== "backup") {
             // don't wait for it to complete
             this._crypto.backupGroupSession(
                 session.room_id,

src/crypto/index.js

@@ -2884,18 +2884,18 @@ Crypto.prototype.backupGroupSession = async function(
     sessionId, sessionKey, keysClaimed,
     exportFormat,
 ) {
-    if (!this.backupInfo) {
-        throw new Error("Key backups are not enabled");
-    }
-
     await this._cryptoStore.markSessionsNeedingBackup([{
         senderKey: senderKey,
         sessionId: sessionId,
     }]);
 
+    if (this.backupInfo) {
         // don't wait for this to complete: it will delay so
         // happens in the background
         this.scheduleKeyBackupSend();
+    }
+    // if this.backupInfo is not set, then the keys will be backed up when
+    // client.enableKeyBackup is called
 };
 
 /**