Ubuntu's crash report triggered by "normal" exception

**Describe the bug**

Setting a shortcut with modifier twice on the same action triggers Ubuntu's crash report - supposedly because of a "normal" foreach in the logs. Here's what the logs say when running guake in a terminal when the crash report appears :

```
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/guake/prefs.py", line 1403, in each_key
raise Exception('This is ok, we just use it to break the foreach loop!')
Exception: This is ok, we just use it to break the foreach loop!
```

**Expected behavior**

Either a window opens up, telling me the shortcut is already assigned, or nothing special happens and the shortcut gets set correctly.

**Actual behavior**

(Note : In my case, I used CTRL+Q)
A window opens up, saying :

> Error setting keybinding. The shortcut "q" is already in use.

When clicking the Close button, Ubuntu reports an internal crash.

**To Reproduce**

1. Set the shortcut for `Toggle Guake Visibility` to CTRL + (Control modifier seems important)
2. Set the shortcut **again** for the same combination, CTRL +
3. Click "Close" on the "Error setting keybinding" window.

-------------------------------------------------------------------------------

$ guake --support

Guake Version: 3.6.3

Vte Version: 0.60.3

Vte Runtime Version: 0.60.3

--------------------------------------------------
GTK+ Version: 3.24.20

GDK Backend: GdkX11.X11Display

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Desktop Session: ubuntu

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Display: :0

RGBA visual: True

Composited: True

* Monitor: 0 - BNQ HDMI-2
* Geometry: 1920 x 1080 at 1920, 0
* Size: 621 x 341 mm²
* Primary: True
* Refresh rate: 60.00 Hz
* Subpixel layout: unknown
* Monitor: 1 - CMN eDP-1
* Geometry: 1920 x 1080 at 1920, 1080
* Size: 344 x 193 mm²
* Primary: False
* Refresh rate: 60.01 Hz
* Subpixel layout: unknown
* Monitor: 2 - IVM HDMI-1
* Geometry: 1920 x 1080 at 0, 0
* Size: 598 x 336 mm²
* Primary: False
* Refresh rate: 60.00 Hz
* Subpixel layout: unknown