Add better explanation to error message

[odersky]

Fixes #18657

[odersky]

How I fixed it

It was easy (I did not expect it to be hard):

• Compile the program with the unclear error message with -Xprompt, followed by abort. This gives the location where the error was produced.
• Find out what explanations would have clarified the error, and what the values containing this info are. To be sure, I added a test println with the suspected valuable info before the error report call.
• Once I was happy with the kind of additional info, I changed the error message to take these additional infos as parameters. If the error had been reported by a simple string, without going through an error message in messages.scala, now would have been the time to create that message. One needs to make sure to create an ErrorMessageId as well in this case.
• Add an explain section to the error message.
• Add a test starting with //> using options -explain and a check file to make sure we have the right info.

That's it. It would be great if others would try to do the same thing when they see an obscure error message. Create an issue first and then fix it according to these steps. These are incremental improvements but they are really valuable in everyday work!

[bishabosha]

Maybe the reader can infer from looking at the printed tree, but perhaps also explain that placeholder parameters always infer parameters 1 level of nesting up from their position (however best to explain that).

[odersky]

Indeed, not sure how to explain that, in a situation like this, where context is missing.