setrounding uses old-style warn and error functions

[ssfrr]

This is on both master and the nonrecursive_powers branch.

https://github.com/JuliaIntervals/IntervalArithmetic.jl/blob/nonrecursive_powers/src/intervals/rounding.jl#L315-L319

I hit this because I was calling setrounding(Interval, :none) in my main package body instead of from an __init__() function, which causes issues during precompilation.

[Kolaru]

setrounding has been removed on master.