mega: fix crash when logging in with previous auth keys fails

When Mega.LoginWithKeys() fails to make the API request, it leaves the
object in a state where Mega.FS.root is nil because it could never query
any information about the filesystem tree. An easy way for this to
happen is if the device is not connected to the internet.

Previously, these failures would be ignored, but Fs.findRoot() on the
rclone side is written in a way that assumes the go-meta filesystem will
have a non-nil root. This leads to an immediate nil pointer dereference
when NewFs() calls Fs.findRoot().

This commit fixes the problem by making LoginWithKeys() failures hard
failures, similar to the MultiFactorLogin() path.

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

backend/mega/mega.go

@@ -284,7 +284,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 			}
 			err = srv.LoginWithKeys(opt.SessionID, decodedMasterKey)
 			if err != nil {
-				fs.Debugf(f, "login with previous auth keys failed: %v", err)
+				return nil, fmt.Errorf("login with previous auth keys failed: %w", err)
 			}
 		}
 	}