[Docs] Make tox -e docs fail on sphinx warnings

* Also cleanup isort command

tox.ini

@@ -191,8 +191,8 @@ commands =
     echo -e "sphinx-apidoc --force -o docs/source/modules/ deluge deluge/plugins"\
         "\n""sphinx-build -v -E -T -b html -d docs/build/doctrees docs/source docs/build/html"
     python -c "import subprocess, sys; proc = subprocess.Popen(\
-        'python setup.py build_docs', shell=True, stdout=subprocess.PIPE);\
-        out, err = proc.communicate(); print err; sys.exit(bool(err))"
+        'python setup.py build_docs', shell=True, stderr=subprocess.PIPE);\
+        err = proc.communicate()[1]; print err; sys.exit(bool(err))"
 
 [testenv:docscoverage]
 sitepackages = {[docsbase]sitepackages}