Improved error messages for plot addition.

[clauswilke]

I made a few improvements to the error messages when plot addition fails, based on recommendations in the tidyverse style guide and also based on feedback from this Twitter thread: https://twitter.com/ClausWilke/status/1189725947508989953?s=20

library(ggplot2)

ggplot() + 1
#> Error: Can't add `1` to a ggplot object.

ggplot() + geom_point
#> Error: Can't add `geom_point` to a ggplot object.
#> Did you forget to add parentheses, as in `geom_point()`?

^{Created on 2019-10-30 by the reprex package (v0.3.0)}

[yutannihilation]

Looks good. I personally liked the simpleness of "Did you mean," but, looking at your Twitter thread, it seems better to hint about parentheses explicitly.

[lock]

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