stat_binhex() fails to notify users that they need to install hexbin package

[yutannihilation]

#4671 (comment)

I can reproduce this on my local without hexbin installed.

> library(ggplot2)
> d <- ggplot(diamonds, aes(carat, price))
> d + geom_hex()
Warning message:
Computation failed in `stat_binhex()`:

I think this is because check_installed is inside compute_group, where the error is ignored.

ggplot2/R/stat-binhex.r

Line 52 in c89c265

check_installed("hexbin", reason = "for `stat_binhex()`")

[yutannihilation]

Related: #4843

[yutannihilation]

Resolved by #4856