[Tests] fix `nvm install-latest-npm` tests to fail properly

.travis.yml

@@ -30,7 +30,7 @@ script:
   - if [ -n "${SHELL-}" ] && [ -n "${TEST_SUITE}" ]; then if [ "${TEST_SUITE}" = 'installation_iojs' ]; then travis_retry make TEST_SUITE=$TEST_SUITE URCHIN="$(npm bin)/urchin" test-$SHELL ; else make TEST_SUITE=$TEST_SUITE URCHIN="$(npm bin)/urchin" test-$SHELL; fi; fi
 before_cache:
   - if [ -n "$WITHOUT_CURL" ]; then sudo apt-get install curl -y ; fi
-  - if [ -n "${NODE-}" ]; then . nvm.sh && nvm install --latest-npm "${NODE}" && npm --version; fi
+  - if [ -n "${NODE-}" ]; then . nvm.sh && set -ex && nvm install --latest-npm "${NODE}" && npm --version; fi
 env:
   global:
     - CXX=g++-4.8