mask splits into (r, g, b) but not (r, g, b, a) causing the program to crash

[durandarthur]

cause of issue: https://github.com/mist-project/mist/blob/67de001bb6a24edeb0ad4c9ebd4225136ddc8089/mist-webui.py#L12

steps to reproduce: use webui

suggestion: replace "r, g, b, a" by "r, g, b"