cmd/go: silence standard imports non-standard error for non-existent import target

This error only affects the compilation of the standard library,
but I discovered that if you import "notexist" from the standard
library then you get both an error about notexist not existing
and an error about notexist being a non-standard package
(because the non-existant package is in fact not a standard package).
Silence the second error.

Change-Id: Ib4c1523e89844260fde90de3459ec1e752df8f25
Reviewed-on: https://go-review.googlesource.com/19383
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/pkg.go

@@ -967,7 +967,7 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 				}
 			}
 		}
-		if p.Standard && !p1.Standard && p.Error == nil {
+		if p.Standard && p.Error == nil && !p1.Standard && p1.Error == nil {
 			p.Error = &PackageError{
 				ImportStack: stk.copy(),
 				Err:         fmt.Sprintf("non-standard import %q in standard package %q", path, p.ImportPath),