serve: Improve Jinja rendering error reporting. Some errors in Jinja are not wrapped in `TemplateSyntaxError` exceptions, so we have to wrap them ourselves to at least say what file it happened in.

sudo: false
language: python
python:
  - "3.4"
install:
  - "pip install -r requirements.txt"
  - "pip install -r dev-requirements.txt"
script: py.test