Error signing in on MS Edge on Windows Mobile

**Describe the bug**
When signing in into Hydrogen on Windows 10 Mobile using MS Edge 40.15252.490.0 (aka EdgeHTML 15), the following error occurs directly on the login screen disallowing any further use:

```
Something went wrong: Object doesn't support property or method 'finally'.
```

I tried using two different Synapse servers, hence it seems to be due to Hydrogen on old Edge and not due to miss-configured Servers

**To Reproduce**
Steps to reproduce the behavior:
1. Go to 'https://hydrogen.element.io'
2. Insert credentials and click on 'Log In'
3. See error

**Expected behavior**
*Well, obviously, it should sign me into my Matrix account.*

**Screenshots**

**Smartphone:**
- Device: Microsoft Lumia 640 XL
- OS: Windows 10 Mobile armhf final 1709, 10.0.15254.490
- Browser Microsoft Edge
- Version 15

**Additional context**
*Unfortunately, Hydrogen in MS Edge would be the only way to connect to the Matrix on Windows Mobile devices. That's why it is really necessary to have at least Hydrogen working as Element understandably won't work on such evil MS devices.*