Throw more informative error message for grid graphics objects

[ecoRoland2]

This came up on Stackoverflow: https://stackoverflow.com/q/78865088/1412059

Users often don't realize that they are dealing with grid graphics, e.g., because they might have simply used a plot method from some package. This can lead to code like this which throws an uninformative error:

library(grid)
library(rvg)
library(officer)

p <- grid.rect(gp = gpar(fill = "red"), draw = FALSE)

read_pptx() |>
  add_slide(layout = "Title and Content", master = "Office Theme") |>
  ph_with(dml(code = {
   p
  }), 
  location = ph_location_type(type = "body"))
#Error in read_xml.raw(charToRaw(enc2utf8(x)), "UTF-8", ..., as_html = as_html,  : 
#  StartTag: invalid element name [68]

It would be nice if the error message could give users a hint that they might have a grid graphics object and should try plot(p). Even better if ph_with.dml could automagically do that for the user.

[davidgohel]

it should be better now