don't mask build failures in Makefile

The pipe to tee was discarding the exit status from python setup.py
build which means that errors were ignored (although most errors would
trigger a proper failure later on during rpmbuild). The grep'ing for
ImportError stuff is obsolete anyway, since docs are now built
separately, so just remove the pipe entirely.

Change-Id: I0bf1752ec3f9c6346dea89b2f801da0f49820a01