Unknown theme elements throw warnings instead of errors

[teunbrand]

This PR aims to fix #5719.

Briefly, when declaring an unknown theme element, we throw a warning instead of error. The offending theme element is then removed from the theme.

This is more gentle if you make a typo or for us if we want to deprecate theme arguments.

Reprex from the issue:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  theme(foobar = "qux")
#> Warning in plot_theme(plot): The `foobar` theme element is not defined in the
#> element hierarchy.

^{Created on 2024-03-05 with reprex v2.1.0}

[thomasp85]

Apart from the comment LGTM

[thomasp85]

Wouldn't this require an update of the snapshot

[teunbrand]

The message is exactly the same so the snapshot file hasn't changed

[thomasp85]

👍