Fix error message from linting a docx file with invalid syntax

[ColonelThirtyTwo]

get_undeclared_template_variables processes the template but wasn't wrapped in map_jinja2_render_errors, so syntax errors weren't being caught properly and failing with a generic failure. This patch fixes it to throw correctly and show the error to the user.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.95%. Comparing base (a663368) to head (3660bfe).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #470 +/- ## ======================================= Coverage 91.95% 91.95% ======================================= Files 310 310 Lines 18102 18102 ======================================= Hits 16645 16645 Misses 1457 1457 ```

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