misleading error message thrown by `cut_interval()`

[djnavarro]

cut_interval() throws a misleading error message when the user does not specify exactly one of n or length. In the worst case, if the user supplies width instead of length (as they would for cut_width(), you get a very fun error 😀

ggplot2::cut_interval(x = 1:10, width = 10)
#> Error: Specify exactly one of n and width

^{Created on 2021-05-03 by the reprex package (v0.3.0)}

The error gets thrown by breaks() here. Looks like an easy fix. Happy to send a PR if you don't want to bother doing it yourself 🙂

[thomasp85]

Please do, thanks 🙂🙏