Return processing information from pdflatex() and friends

[krlmlr]

also in case of failures. Use case: tikzDevice.

Something like:

structure(
  "path/to.pdf",
  log_path = "path/to.log",
  exec_time = 1.5,
  success = TRUE,
  ...,
  class = c("pdflatex_tinytex_output", "tinytex_output")
)

We also need a way to specify that we always return this info regardless of success or failure, current behavior should be the default.