Move error function from osu.Game.Utils to osu.Game.Rulesets.Difficulty.Utils

[Natelytle]

This is only ever going to be used for difficulty calculation anyway, so we might as well keep all diffcalc related files in one spot.

[peppy]

Please squash your commits (force pushing is fine). We don't need such commit messages in history thanks.

To clarify, force pushing is preferred to stupid commits if no one has reviewed the PR yet.

[stanriders]

Please squash your commits (force pushing is fine). We don't need such commit messages in history thanks.

To clarify, force pushing is preferred to stupid commits if no one has reviewed the PR yet.

pp-dev merging policy is squash-only

[tsunyoku]

I think the filename of the error function file should be a bit clearer to the fact it belongs to the DifficultyCalculationUtils class. Following general partial class naming conventions, could we rename to DifficultyCalculationUtils_ErrorFunction.cs?

Otherwise looks good.