How to trigger a custom error from inside a Jinja template?

Though Ansible itself has a way for triggering a custom error, I can not find anything similar for Jinja.

My current method uses a syntax error:

{%  if 'ansible_mounts' in hostvars[host] %}
# {{ host }} knows its mount-points
{% else %}
# {% error!! No ansible_mounts listed for host - fact-gathering must've failed %}
{% endif %}

but those are rendered poorly at run-time -- one needs to look inside the template-file and search for the error (the rendering does not even include the line-number!).

Is there a way to output a neat failure message from inside Jinja-template?

Solution 1:

Answer from the comment:

There is no easy way to accomplish this – only via custom plugin. For details see: https://stackoverflow.com/questions/21778252/how-to-raise-an-exception-in-a-jinja2-macro

Solution 2:

Ansible adds the mandatory filter to Jinja, which can be used to do this:

{{ ('OK text' if condition_ok) | mandatory('Text of error message') }}

gives the failure:

fatal: [hostname]: FAILED! => {"msg": "Text of error message"}

(Replace condition_ok with the check that you need to make; 'OK text' can be just '' if nothing should be substituted.)