gdbgui --rr crashes if there is no previous recording

**Description**

Without having recorded something in advance (`rr record`) gdbgui just crashes on startup:

```
set breakpoint pending on
The underlying gdb process has been killed. This tab will no longer function as expected.
Error occurred on server when running gdb command: gdb is not running
```

Obviously such usage is not advised and that's something an experienced `gdb`, `rr` user wouldn't do, but may discourage new users who would like to use this functionality.

**To Reproduce**

Run `gdbgui --rr` without having pre-recorded anything.

**Please complete the following information:**
* OS: Linux Ubuntu 16.04
* gdbgui version (`gdbgui -v`): 0.13.1.1
* gdb version (`gdb -v`): GNU gdb (Ubuntu 8.2-0ubuntu1~16.04.1) 8.2
* browser: Firefox

P.S. Thanks for this amazing tool.