Improve error handling in fj_export

[chrisprice]

Thanks for the feedback on #594, I'm always keen to learn. This PR attempts to address the comments. Please feel free to keep the comments flowing!

[hannobraun]

Thank you for the follow-up, @chrisprice!

This looks excellent! I left some comments with suggestions, but this is some high-level complaining I'm doing there. No need to make any changes, but of course, another follow-up PR is welcome, if you're so inclined.

[hannobraun]

The Option<String> in the error can make for a very confusing error message. How about changing it to String, and using something like this here instead:

Suggested change

	extension.to_str().map(|s| s.to_string()),
	extension.to_string_lossy().into_owned(),

[hannobraun]

Copy-pasting the error message works for the other doc comments, but not this one 😁

Suggested change

	/// Unrecognised extension found `{0:?}`
	/// Unrecognised extension found

[hannobraun]

Hmm, I changed the PR title to highlight the addition of Triangle::normal, but on second thought, I'll pull that into a separate PR. Triangle::normal and fj_export::Error are both additions to the public API. But since they are in different crates, they need to be highlighted in different sections of the Weekly Dev Log and the release changelog.

This is not a complaint about this pull request, I'm just explaining what I'm doing. Feel free to split future pull requests accordingly, if you happen to remember, but otherwise don't worry. Keeping track of this organization stuff is my job 😁

[hannobraun]

Rebased on top of #600. Merging.