encoding/xml: minor improvement to namespace mismatch in closing tag

[candlerb]

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

play.golang.org (1.17.2)

Does this issue reproduce with the latest release?

Yes

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

N/A

What did you do?

https://play.golang.org/p/2J4gNo_saN3

What did you expect to see?

Error message about closing tag being in wrong namespace

What did you see instead?

The error message is accurate:

panic: XML syntax error on line 4: element <foo> in space http://example.comclosed by </foo> in space 

However there are a couple of ways this could be improved.

1. Add a whitespace separator before the words "closed by", so it doesn't crash into the namespace URI
2. Put quotation marks around the namespace (or make the null namespace visible in some other way; or remove the words "in space" if the namespace is not set).

e.g. this would be better:

panic: XML syntax error on line 4: element <foo> in space "http://example.com" closed by </foo> in space ""

Reference: https://github.com/golang/go/blob/master/src/encoding/xml/xml.go#L502

[candlerb]

Here's an example of a different place which gives a better error message, saying "no name space" where there isn't one:
https://play.golang.org/p/uOWeDoGXW2g

panic: expected element <Edmx> in name space http://docs.oasis-open.org/odata/ns/edmx but have no name space

(and I think "name space" is clearer than just "space")

[gopherbot]

Change https://go.dev/cl/535535 mentions this issue: encoding/xml: display closing space in error message