#133 Buildscript improvements

This includes two commits. One stops the buildscript from doing weird stuff when the venv-creation fails (should solve #113), the other commit improves build times by not doing a full rebuild (including recreation of the venv) unless it is requested with the new "--full" parameter. Due to not deleting the venv on every build, this should also resolve #126. Tested on Arch and macOS 10.12.