Refactor weapon turret setup validation

[lL1l1]

Issue

When turret setup fails it does not give very clear information because the error messages are vague or the stack trace is incorrect due to bone validation itself erroring. Most commonly this occurs when developing mods.

Description of the proposed changes

• Refactors turret validation so the issues are clear.
• Change turret setup failure from error to return.
• Change Unit:ValidateBone so it doesn't error there, and instead just returns false.
• Fix ValidateBone not dealing with nil bones, a common case when turrets are setup incorrectly (forgetting a bone).
• Format weapon.lua

Testing done on the proposed changes

Remove bones for a unit's turret setup and check the new warning message.

Checklist

• Changes are annotated, including comments where useful
• Changes are documented in the changelog for the next game version