Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

golang.org goplayground (1.8 I believe)

What operating system and processor architecture are you using (go env)?

I am using go playground from Firefox.

However for completeness:
GOARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
Firefox 53.0.2 (64-bit)

What did you do?

https://play.golang.org/p/NtvSPO4xx7

What did you expect to see?

An error message like:
"Badly formed Rune" or "Bad rune value" maybe even
"runes with values larger than 255, 0xFF, can't be defined using multiple \xhh values"

What did you see instead?

tmp/sandbox523565836/main.go:5: missing '
tmp/sandbox523565836/main.go:5: illegal character U+005C '\'
tmp/sandbox523565836/main.go:5: syntax error: unexpected xb8 at end of statement
tmp/sandbox523565836/main.go:5: newline in character literal

Issue Explanation/Justification

I think the Go compilers error messages are unnecessarily unhelpful or confusing.

The source doesn't look like there is a "missing ' ", it looks like there are a matching pair of single quotes.
The character U+005C '\' is legal in rune values, so that seems confusing.
In this example, the value xb8 is apparently in the middle of the rune, and so "syntax error: unexpected xb8 at end of statement" seems confusing.
The line doesn't look like there is a "newline in character literal".

A plausible behaviour might be to 'collect' all the characters up to and including an unescaped closing single quote (on the same line, and before a '//' comment mark), and treat all of those characters as part of the bad rune value.