Handle incorrect billable? value in client

[FreekPaans]

You can't mark an entry as billable when task billing method is :fixed-price or :overhead, enforce this in frontend.

[jeroenheijmans]

Ah, yeah, I encountered an issue where I posted an entry but got his back:

HTTP ERROR: 500

Problem accessing /entry/2017-04-18. Reason:

Assert failed: entry can only be billable when task billing method in #{:hourly :hourly}
(valid-billable? value for-task)

Caught me by surprise :-)

(Seems to be the same issue, just wanted to add some detail.)