Make deprecated IO.onError package private

[CJSmith-0141]

Also adds a test to make sure that code outside the package can still compile with a total function.

Closes #4329

Thank you @armanbilge for the solve and @Grogs for the writeup.

[CJSmith-0141]

Looks like making it package private doesn't work for 2.12. Maybe un deprecate instead?

[joroKr21]

I think on 2.12 you can't use a total function in place of a partial function. So that test would have to be only for 2.13+

[CJSmith-0141]

Ok since this would be a source breaking change for Scala 2.12, I changed the base branch for this PR to be series/3.x.

I also moved the test to be 2.13+.

[joroKr21]

The previous change was already source breaking because Scala 2.12 can't infer the parameter type of the function passed to an overloaded method like onError { case e => ... } will complain about unknown function parameter type.

[armanbilge]

thanks!

[djspiewak]

@armanbilge Are we going to retarget this to 3.6?

[armanbilge]

Are we going to retarget this to 3.6?

Yes!

[armanbilge]

superseded #4343