For reference, go/types prints

runes.go:5:9: illegal rune literal

We could mimic go/types behaviour. I have no idea how much work would be needed, so leaving it to others to decide if it's worth it.

I'd be satisfied with "illegal rune literal". One message, terse, accurate.

As an aside ...
Is their a 'place' explaining all of the Go compilers error messages? My search didn't find it.
It would be nice to have all Go compiler error messages along with a brief explanation of a possible cause of the error in a web-searchable place.
I paste error messages from software into a web browser's search box, but in this case it didn't find much help. Sometimes stackoverflow is helpful, but not in this case.
Explaining compiler error-messages might be a straightforward, well-bounded, activity to ask the Developer Experience Working Group to solve. The result could be helpful for new or occasional Go programmers, and so help them climb the learning curve more quickly and effectively.
Further, if the 'place' had some authority or evidence of good quality, it might be the 'go to' place.

I apologise for not understanding this, but why add it to Go1.10 milestone? Didn't that happened years ago?

Is their a 'place' explaining all of the Go compilers error messages

I don't think there is. I know Rust has something like that; and I agree that it could be nice to have something similar (even though Go is a simpler language and error messages usually are easy to understand; I rarely see people puzzled by a compiler error when programming in Go).

Explaining compiler error-messages might be a straightforward, well-bounded, activity to ask the Developer Experience Working Group to solve

I agree. You could ask for this on their mailing list. I don't think it's "straightforward", though. Doing this (and doing it well) will require a fair amount of effort.

I apologise for not understanding this, but why add it to Go1.10 milestone? Didn't that happened years ago?

The dot in the versioning number is not a decimal point, i.e. 1.10 is not 1.1. The sequence goes 1.1, 1.2, ..., 1.9, 1.10, 1.11, etc...

The next go version will be go1.9, and then we'll have go1.10. I can't assign this issue to the 1.9 milestone because the go1.9 development phase is finished (we're now in a code freeze); so this will be likely considered for go1.10 (unless someone decides that this is critical and needs to be fixed for go1.9).

Thank you for your very swift, clear and useful responses.

I'll think about asking on the mailing list. When I do, I have a small, but hopefully useful, additional idea.

I'd understood the 'dot' in the version is not a decimal point.
I hadn't known milestone names don't align with Go versions. So that is helpful to know.

It seems a very low-priority to have version and milestone names align. I imagine retaining flexibility to have them be different probably significantly outweighs any small gains.

Marking for 1.9 - easy enough to fix.

I'm going to close this. The 1.9 compiler already produces a nice error message. For

$ cat x.go
package main

func main() {
	r20 := '\xe4\xb8\x96'
}

we get

$ go tool compile x.go
x.go:4:9: invalid character literal (more than one character)

That is, this is fixed in the latest tools.
The go playground will be updated to 1.9 once 1.9 is released.