net: handle "dns failure" as errNoSuchHost on Plan 9
net: handle "dns failure" as errNoSuchHost on Plan 9

CL 31468 added TestLookupNonLDH, which was failing on Plan 9,
because LookupHost was expecting to return errNoSuchHost
on DNS resolution failure, while Plan 9 returned the
"dns failure" string.

In the Plan 9 implementation of lookupHost, we now return
errNoSuchHost instead of the "dns failure" string, so
the behavior is more consistant with other operating systems.

Fixes #17568.

Change-Id: If64f580dc0626a4a4f19e5511ba2ca5daff5f789
Reviewed-on: https://go-review.googlesource.com/31873
Run-TryBot: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

derekmarcotte/go,cloudflare/go,suncycheng/go,cloudflare/go,akutz/go,polyverse-security/go,akutz/go,golang/go,Cofyc/go,vsekhar/elastic-go,vsekhar/elastic-go,linux-on-ibm-z/go,CAFxX/go,josharian/go,josharian/go,nawawi/go,akutz/go,polyverse-security/go,vsekhar/elastic-go,vsekhar/elastic-go,derekmarcotte/go,nawawi/go,derekmarcotte/go,akutz/go,codestation/go,Cofyc/go,polyverse-security/go,codestation/go,CAFxX/go,linux-on-ibm-z/go,polyverse-security/go,derekmarcotte/go,akutz/go,Cofyc/go,polyverse-security/go,codestation/go,golang/go,ImJasonH/go,suncycheng/go,linux-on-ibm-z/go,goodlang/good,linux-on-ibm-z/go,goodlang/good,josharian/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,josharian/go,josharian/go,josharian/go,polyverse-security/go,ImJasonH/go,nawawi/go,cloudflare/go,akutz/go,derekmarcotte/go,suncycheng/go,golang/go,codestation/go,goodlang/good,vsekhar/elastic-go,ImJasonH/go,derekmarcotte/go,goodlang/good,ImJasonH/go,vsekhar/elastic-go,josharian/go,golang/go,Cofyc/go,linux-on-ibm-z/go,suncycheng/go,akutz/go,josharian/go,vsekhar/elastic-go,ImJasonH/go,suncycheng/go,cloudflare/go,suncycheng/go,nawawi/go,CAFxX/go,polyverse-security/go,golang/go,akutz/go,Cofyc/go,linux-on-ibm-z/go,linux-on-ibm-z/go,CAFxX/go,goodlang/good,golang/go,ImJasonH/go,CAFxX/go,CAFxX/go,golang/go,Cofyc/go,codestation/go,cloudflare/go,codestation/go,Cofyc/go,CAFxX/go,derekmarcotte/go,CAFxX/go,nawawi/go,Cofyc/go,christopher-henderson/Go,goodlang/good,derekmarcotte/go,polyverse-security/go,goodlang/good,codestation/go,linux-on-ibm-z/go,codestation/go,cloudflare/go,suncycheng/go,nawawi/go,ImJasonH/go,suncycheng/go,ImJasonH/go,goodlang/good,vsekhar/elastic-go,cloudflare/go