Rust 1.93 causes TimeDelta::__add to fail when adding a userdata to
itself with a bland "error borrowing userdata" error message.
Go back to 1.92 to unblock builds for the moment.
For this invalid jinja field: `{{ First Name }}` we'd report:
```
syntax error: unexpected identifier, expected end of variable block (in 0:1)
```
which is not very helpful.
With this commit we now report:
```
failed parsing field 'content.text_body' as template: syntax error: unexpected identifier, expected end of variable block (in template '0' line 1: 'Hello {{ First Name }},')
```
which is much more informative!
We include a length-limited fragment of the extracted source line in the
message to help track down the issue. It is length limited because the
templates can potentially be very large.
The string truncation requires a function that was stabilized in rust
1.91, so this commit bumps the minimum version required.
Wez Furlong
Sophia Turner