Enhancement: Show *why* ship is not building when zero productivity

[henrybauer]

Users often don't pay any attention to productivity, then wonder why they can't build anything. The GUI could clue users in to this instead of sitting at 0% forever. Two ideas below, but any GUI feedback would be welcome:

1. If you press "Build" and productivity is zero, pop up an error instead of progressing to the build window

2.The Build window could show a warning when productivity is zero (perhaps instead of the progress)

[taniwha]

Fixed by 9b7ccd7