Improve error messaging in sidebar YAML parser

[pikinier20]

https://github.com/lampepfl/dotty/blob/main/scaladoc/src/dotty/tools/scaladoc/site/SidebarParser.scala
contains only general info about the file format and returns a general error for all problems. The idea is to improve the error handling so that user can see an erroneous line or which information is missing.

[gringrape]

Could I work on this?

[julienrf]

Thank you for your interest, sure, feel free to do so! Do you already have a specific plan in mind?

[gringrape]

Thank you for your interest, sure, feel free to do so! Do you already have a specific plan in mind?

Thank you. Actually, I don't have plan not right now. But, I am interested in this and want to check detail.
Or could you share your idea?

[julienrf]

I don’t have anything in particular off the top of my head: I would try to write incorrect sidebar files, look at the output of scaladoc, and see how to improve it…

[Dedelweiss]

Hello @gringrape, I was going to start correcting this issue but I see that you have started working on it. Is this still the case? Have you started anything?

[Dedelweiss]

I have started a first PR with some questions, if you are interested to see it.