explicitly error out on bad pip install

bin/steps/pip-install

@@ -4,8 +4,16 @@ puts-step "Installing dependencies with pip"
 [ ! "$FRESH_PYTHON" ] && bpwatch start pip_install
 [ "$FRESH_PYTHON" ] && bpwatch start pip_install_first
 
+set +e
+
 /app/.heroku/python/bin/pip install -r requirements.txt --exists-action=w --src=./.heroku/src --allow-all-external --disable-pip-version-check --no-cache-dir | cleanup | log-output | indent
 
+if [[ "$?" ]]; then
+    echo "an error occurred."
+    exit
+fi
+set -e
+
 # Smart Requirements handling
 cp requirements.txt .heroku/python/requirements-declared.txt
 /app/.heroku/python/bin/pip freeze > .heroku/python/requirements-installed.txt