Crash if dependency not found

Hi Andrew,

TinderBox crashes (on Windows) if a block contains a `<requires>` statement that can't be met. I tried the following without having OpenCV installed:

`<requires>org.libcinder.opencv < / requires >`  (without the spaces)

Upon selecting "Copy" or "Relative" from TinderBox's popup menu, the application hangs.

-Paul
