Improve mobile app template handling

[balloob]

Breaking change

Proposed change

Rendering templates via the mobile app was a little cumbersome. Invalid templates would fail the payload and errors in templates would propagate up.

This makes sure we catch them all.

Note, our current template return type is a little problematic, since starting with 117 we will be able to return native values from templates, and so the user could return a similar shaped error object from a template 🤔

CC @zacwest

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[balloob]

@bdraco this is a little weird I think. We need to catch all exceptions because template {{ now() | random }} raises a TypeError that crashes the websocket connection (500 error) and also here had to be caught with a generic error. Should we catch all exceptions that come out of render and wrap it in TemplateError ?

[bdraco]

Wrapping .render in template.py seems like it will also fix some other bugs where templates stop updating because the exception isn't trapped.

I'm doing testing with it now.

[bdraco]

It also looks like {{ now() | rando }} is untrapped.

[bdraco]

I've found a few things. I'll open a new PR to fix them, when its done trapping except TemplateError as ex will do what was originally expected here.

[bdraco]

Fixed in #41719