Fix setup workflow for libuv

If libuv cannot be downloaded because git is missing, the setup
will leave the empty *deps* directory. This will break the following
build even if the user has installed git in the meantime, because
the setup routine assumes that libuv is already downloaded.

With this patch the "deps" directory will be deleted if the download
of libuv fails, leaving pyuv in a clean state where the second 
setup approach will succeed if the git dependency is resolved.