Improve message of palette/na.value mismatch

[teunbrand]

This is a spontaneous PR without a linked issue.

The current error message when using incompatible palette and na.value is confusing. It is unclear why na.value should have the character type.

library(ggplot2)

df <- mpg
df$drv[12] <- NA

ggplot(df, aes(displ, hwy, shape = drv)) +
  geom_point() +
  scale_shape_manual(
    values = c("circle small", "triangle", "square"),
    na.value = 4
  )
#> Error in `vec_slice<-`:
#> ! Can't convert `na_value` <double> to <character>.

^{Created on 2025-12-08 with reprex v2.1.1}

In this PR, it is more clear (it should match the palette type).

#> Error in `vec_assign()` at ggplot2/R/scale-.R:1343:7:
#> ! Can't convert `na.value` <double> to match type of `palette` <character>.