Demote errors in legend modified expressions

[teunbrand]

This PR aims to fix #6264.

When we have modified expressions with e.g. stage() that fail to evaluate, we demote the error to a warning and don't apply these expressions.

[yjunechoe]

Thanks -- this works for my use-case!

I think the try_fetch() might be implemented too loosely here, though, as it seems to also demote genuine evaluation errors with after-scale expressions on the layer data.

p <- ggplot(mpg, aes(drv)) +
  geom_bar(
    aes(fill = after_scale(stop())),
    # show.legend = FALSE
  )

On main:

p
#> Error in `geom_bar()`:
#> ! Problem while setting up geom aesthetics.
#> ℹ Error occurred in the 1st layer.
#> Caused by error in `after_scale()`:

(plot fails to build)

On branch:

p
#> Warning: Unable to apply legend modifications.
#> Caused by error in `after_scale()`:

---

May be out of scope and may not need this as part of the solution, but I wonder whether it's worth considering passing some kind of a flag to Layer$compute_geom_2()/Geom$use_defaults() to indicate whether the data represents the layer geom or the guide.

[teunbrand]

You raise a good point, though IMO the error message is overspecific. If we just say the staged modifications failed, without mentioning legend or layer, we needn't fuss about passing along context.