prompt: Fail on no stories to be offered

Modify prompt.PromptStories to return an error in case an empty stories
slice is passed into it so that the function fails immediately when
someone tries to ask the user to choose from 0 stories.

Change-Id: 9927de2a48
Story-Id: SF-80
review post: Improve user-facing message

Change-Id: d182f53542
Story-Id: SF-66
story start: Skip branch creation when name empty

Skip the branch creation step when the user inserts an empty branch
name. This is basically equivalent to running story start with
-no_branch.

Change-Id: f862519322
Story-Id: SF-81
wip [ci skip]

Change-Id: 1460f1319c
wip [ci skip]

Change-Id: 2cb10aee39
wip [ci skip]

Change-Id: a4a20ced77
wip [ci skip]

Change-Id: 8450704712
wip [ci skip]

Change-Id: 5e058c7e0d
wip [ci skip]

Change-Id: 64c09aa8b2
wip [ci skip]

Change-Id: 173c55c9c7
wip [ci skip]

Change-Id: 3d64934a4e