Azure: Add missing "set -e" to docs job

Without setting the shell flag to exit immediately when a command exists
with a non-zero status we can have the situation where the htmldocs
target fails with an error but the job will succeed due to infodocs
passing and being the last build target.

Signed-off-by: Tom Rini <trini@konsulko.com>

.azure-pipelines.yml

@@ -89,6 +89,7 @@ stages:
       options: $(container_option)
     steps:
       - script: |
+          set -e
           virtualenv -p /usr/bin/python3 /tmp/venvhtml
           . /tmp/venvhtml/bin/activate
           pip install -r doc/sphinx/requirements.txt