Register an rlang_error method for knitr::sew() to handle backtraces in error messages

[lionel-]

This method consults rlang_backtrace_on_error to determine whether to display a backtrace with the error message. Whereas the default value for this option is "reminder" in interactive sessions and "full" in non-interactive sessions, the default is set to "none" in knitr chunks that expect errors (error = TRUE chunks).

The method also sets rlang::last_error(). Consulting the last error must be done in a different chunk since the sew() generic is run at the end of a chunk.

This should fix all corner cases regarding backtrace handling in reprex::reprex(). Closes #1205. cc @jennybc.

I will also look into setting rlang_trace_top_env from knitr. Would there be a downside to do this @hadley?

[lionel-]

I will also look into setting rlang_trace_top_env from knitr. Would there be a downside to doing this?

oh hmm now I remember your idea in #932

We could use knitr::knit_global() to access the chunk environment. I'll look into this tomorrow.

[hadley]

What about errors not captured by knitr?

[lionel-]

As before they fall through the normal case and display a reminder in interactive sessions, or a backtrace in non-interactive sessions:

Quitting from lines 34-36 (test-trace-full.Rmd) 
Error: foo
Backtrace:
    ▆
 1. └─global::f()
 2.   └─global::g()
 3.     └─global::h()