Clarify the missing-Cairo error message
Clarify the missing-Cairo error message

Currently the error is not as helpful as it could be. Add a snippet
that the user could quickly copy-paste when they encouter this message.
Also, make the code a bit more readable.

Hydrotoast/Compose.jl,jiahao/Compose.jl,JuliaPackageMirrors/Compose.jl,dcjones/Compose.jl