fortify.grouped_df should have a more informative error message if dplyr is not installed

[jimhester]

The current behavior just gives you a very uninformative error message, particularly because it references dplyr when you may not even be using dplyr in your code! e.g. https://travis-ci.org/adamhsparks/theme.usq/builds/414441396#L3184

# remove.packages("dplyr")
library(ggplot2)
p6 <- ggplot(economics_long) +
  aes(date, value01, colour = variable) +
  geom_line()
#> Error in loadNamespace(name): there is no package called 'dplyr'

Created on 2018-08-10 by the reprex package (v0.2.0).

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