More useful error display when stderr is empty

I tried to use Cocoduino with Arduino 1.5.2 beta. boards.txt is in a new location (actually two new locations, one per architecture) in that version, so ino couldn't find it. When ino itself fails, it emits no standard-error output; instead, it dumps its error message to stdout.

These changes make Cocoduino use the stdout string when the error string is empty, so that the error sheet is not empty if Cocoduino can help it, and provide a specific error message for the case of missing boards.txt (i.e., Arduino IDE version that Cocoduino doesn't recognize).