Better error handling when using invalid icon

[svenove]

Is your feature request related to a problem? Please describe. If trying to use a non-existing icon, the drawcustom fails with a confusing error: "invalid literal for int() with base 16: ''

Describe the solution you'd like Check that an icon exists and give an understandable error message when it’s doesn’t.

[jonasniesner]

will be fixed in the release this evening