cmd/go: go list -e -compiled -json does not populate Error when cgo preprocessing fails

[dominikh]

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

$ go version
go version go1.14.1 linux/amd64
go version devel +57e32c4fbd Sun May 10 14:43:46 2020 +0000 linux/amd64

What's wrong

When cgo preprocessing fails, go list -json -compiled -e prints an error on stderr and exits with a non-zero status, but it does not populate the Error field of the package. It is possible to detect that something went wrong because CompiledGoFiles isn't populated, but we can't tell why it failed. Two common reasons are a missing C compiler, and missing libraries.

Example output

$ CC=/does/not/exist go list -json -compiled -e runtime/cgo
# runtime/cgo
fork/exec /does/not/exist: no such file or directory
{
	"Dir": "/nix/store/sylhlk3fd3xik0d8hqrm20zxnfc3x7m4-go-1.14.1/share/go/src/runtime/cgo",
	"ImportPath": "runtime/cgo",
	"Name": "cgo",
	"Doc": "Package cgo contains runtime support for code generated by the cgo tool.",
	"Target": "/nix/store/sylhlk3fd3xik0d8hqrm20zxnfc3x7m4-go-1.14.1/share/go/pkg/linux_amd64/runtime/cgo.a",
	"Root": "/nix/store/sylhlk3fd3xik0d8hqrm20zxnfc3x7m4-go-1.14.1/share/go",
	"Match": [
		"runtime/cgo"
	],
	"Goroot": true,
	"Standard": true,
	"Stale": true,
	"StaleReason": "build ID mismatch",
	"GoFiles": [
		"callbacks.go",
		"callbacks_traceback.go",
		"iscgo.go",
		"mmap.go",
		"setenv.go",
		"sigaction.go"
	],
	"CgoFiles": [
		"cgo.go"
	],
	"IgnoredGoFiles": [
		"callbacks_aix.go",
		"dragonfly.go",
		"freebsd.go",
		"netbsd.go",
		"openbsd.go",
		"signal_darwin_armx.go"
	],
	"CFiles": [
		"gcc_context.c",
		"gcc_fatalf.c",
		"gcc_libinit.c",
		"gcc_linux_amd64.c",
		"gcc_mmap.c",
		"gcc_setenv.c",
		"gcc_sigaction.c",
		"gcc_traceback.c",
		"gcc_util.c"
	],
	"HFiles": [
		"libcgo.h",
		"libcgo_unix.h"
	],
	"SFiles": [
		"asm_amd64.s",
		"gcc_amd64.S"
	],
	"CgoCFLAGS": [
		"-Wall",
		"-Werror"
	],
	"CgoLDFLAGS": [
		"-lpthread"
	],
	"Imports": [
		"C",
		"unsafe"
	],
	"Deps": [
		"unsafe"
	]
}

Related: #34229 and #25842

/cc @bcmills @matloob @heschik

[jayconrod]

This has the same root cause as the two linked issues: #34229 and #25842

Currently, go list -e populates the Error field with errors it encounters when loading the package dependency graph. It should also populate the Error field with errors encountered when running cgo, the compiler, and other commands.