Convert C++ exceptions to R conditions

[davidchall]

When rendering template, catch inja.exception.render_error and convert to R condition using rlang::abort(). This condition can store the reason, the missing data variable, and the location (line no, char no). From this metadata, we can construct a very helpful error message.

[davidchall]

Design:

• In C++ code
  • try { ... } catch (const inja::InjaError& e) { ... }
  • extract metadata (type, message, line, column)
  • rethrow as R condition
• In R code
  • catch R condition
  • attach original text
  • rethrow with call info

Using original text and line/column metadata, we can print the line and highlight the point where the problem occurred.

[davidchall]

Related: pantor/inja#249