Checks for bandwidth in stat_density_2d()

[teunbrand]

This PR aims to fix #6374

Briefly, it introduces some fallbacks and more informative error messages for calculating the bandwidth in stat_density_2d().

The reprex from the issue now runs without errors, due to fallbacks when h = 0 to use stats::bw.nrd0() instead of MASS::bandwidth.nrd().

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
set.seed(42)
df <- data.frame(
  x = sample(0:10, 100, replace = T), 
  y = c(rep(5, 80), sample(0:10, 20, replace = T))
)
ggplot(df, aes(x, y)) + stat_density_2d()

Moreover, if the bandwidth is nonsense it will throw an error (demoted to warning by stats). Also noteworthy here is that the contouring step is skipped instead of throwing errors because there is no computed data from the 2D KDE step.

ggplot(df, aes(x, y)) + stat_density_2d(h = c(0, NA))
#> Warning: Computation failed in `stat_density2d()`.
#> Caused by error in `precompute_2d_bw()` at ggplot2/R/stat-density-2d.R:185:5:
#> ! The bandwidth argument `h` must contain numbers larger than 0.
#> ℹ Please set the `h` argument to stricly positive numbers manually.

^{Created on 2025-03-21 with reprex v2.1.1}

[thomasp85]

LGTM