exercises(triangle): simplify error set

[booniepepper]

I took an editing pass of the triangle exercise

• exercises(triangle): TriangleError.InvalidInequality is a weird name #215 Replaced the TriangleError.InvalidEquality with TriangleError.Invalid
• exercises(triangle): Offering private function stubs is weird #216 Removed private fn stubs in the initial code @ee7 is too fast lol
• Misc small refactors to the example

Closes: #215

[ee7]

@ErikSchierboom Any thoughts on what to do here?

[ErikSchierboom]

which is an argument for just one error name. Otherwise the user has to understand when to return Degenerate, without instruction. If we keep the distinction, I think we should explain in an instructions_append.md.

I think I would prefer just one error name. As your research shows, we removed degenerate from the instructions, so my preference would be to not add it here.

[ee7]

Thanks!