cmd/go: report loading errors from 'go mod tidy' and 'go mod vendor'

Fixes #27063

Change-Id: Iedd14fd614a3d79d1387b923a0f123c2bc9e0b33
Reviewed-on: https://go-review.googlesource.com/c/go/+/188763
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/modload/load.go

@@ -387,11 +387,13 @@ func loadAll(testAll bool) []string {
 
 	var paths []string
 	for _, pkg := range loaded.pkgs {
-		if e, ok := pkg.err.(*ImportMissingError); ok && e.Module.Path == "" {
-			continue // Package doesn't actually exist.
+		if pkg.err != nil {
+			base.Errorf("%s: %v", pkg.stackText(), pkg.err)
+			continue
 		}
 		paths = append(paths, pkg.path)
 	}
+	base.ExitIfErrors()
 	return paths
 }
 

src/cmd/go/testdata/script/mod_tidy_error.txt

@@ -0,0 +1,39 @@
+env GO111MODULE=on
+
+# Regression test for golang.org/issue/27063:
+# 'go mod tidy' and 'go mod vendor' should not hide loading errors.
+
+! go mod tidy
+stderr '^issue27063 imports\n\tnonexist: malformed module path "nonexist": missing dot in first path element'
+stderr '^issue27063 imports\n\tnonexist.example.com: cannot find module providing package nonexist.example.com'
+stderr '^issue27063 imports\n\tissue27063/other imports\n\tother.example.com/nonexist: cannot find module providing package other.example.com/nonexist'
+
+! go mod vendor
+stderr '^issue27063 imports\n\tnonexist: malformed module path "nonexist": missing dot in first path element'
+stderr '^issue27063 imports\n\tnonexist.example.com: cannot find module providing package nonexist.example.com'
+stderr '^issue27063 imports\n\tissue27063/other imports\n\tother.example.com/nonexist: cannot find module providing package other.example.com/nonexist'
+
+-- go.mod --
+module issue27063
+
+go 1.13
+
+require issue27063/other v0.0.0
+replace issue27063/other => ./other
+-- x.go --
+package main
+
+import (
+	"nonexist"
+
+	"nonexist.example.com"
+	"issue27063/other"
+)
+
+func main() {}
+-- other/go.mod --
+module issue27063/other
+-- other/other.go --
+package other
+
+import "other.example.com/nonexist"

src/cmd/go/testdata/script/mod_vendor.txt

@@ -171,12 +171,6 @@ package m
 
 import _ "appengine"
 import _ "appengine/datastore"
--- nonexistent.go --
-// +build alternatereality
-
-package m
-
-import _ "nonexistent.rsc.io"
 -- mypkg/go.mod --
 module me
 -- mypkg/mydir/d.go --