fix: Raise ValueError for invalid start_time in create()#264

Marenz merged 2 commits intofrequenz-floss:v1.x.xfrequenz-floss/frequenz-client-dispatch-python:v1.x.xfrom

Marenz:fix-create-invalid-start-timeMarenz/frequenz-client-dispatch-python:fix-create-invalid-start-timeCopy head branch name to clipboard