Build modules: a failed command's reason says its error, not its last progress line (issue 1703)

buildah writes STEP lines on stdout and its error on stderr, so the last line of both hid every image build's cause.
mesh-build-image's reason is now stderr's last lines (600 bytes) with the exit status, and the journal gets both tails;
mesh-build-node keeps stderr's end and stdout's start apart; mesh-build-go's cause skips go build's '# package' line.

modules/mesh-build-go/cmd/mesh-build-go/fetch.go

@@ -331,10 +331,11 @@ func (m *Module) clock() time.Time {
 func (m *Module) now() time.Time { return m.clock() }
 
 // causeLine is the first line of a go command's output that says something other than Go's own progress: `go:
-// downloading ...` comes first when Go fetches a module, and the cause of a failure on a later line.
+// downloading ...` comes first when Go fetches a module, and `# <package>` heads a compile error of go build or go vet
+// (issue 1703: the reason said only the package), and the cause of a failure on a later line.
 func causeLine(s string) string {
 	for _, l := range strings.Split(s, "\n") {
-		if l = strings.TrimSpace(l); l != "" && !strings.HasPrefix(l, "go: downloading ") {
+		if l = strings.TrimSpace(l); l != "" && !strings.HasPrefix(l, "go: downloading ") && !strings.HasPrefix(l, "# ") {
 			return l
 		}
 	}

modules/mesh-build-go/cmd/mesh-build-go/fetch_test.go

@@ -302,3 +302,16 @@ func TestWorkAndCacheAreTheDirectoriesTheManifestPlacesOutsideTheHome(t *testing
 		}
 	}
 }
+
+// issue 1703: a go command's reason is its cause, not Go's progress nor go build's `# <package>` heading.
+func TestCauseLineSkipsProgressAndThePackageHeading(t *testing.T) {
+	for in, want := range map[string]string{
+		"go: downloading x v1\n# git.example/m/cmd\n./main.go:3:2: undefined: y\n": "./main.go:3:2: undefined: y",
+		"\ngo: example.com/x@v1: reading: 404\n":                                   "go: example.com/x@v1: reading: 404",
+		"go: downloading x v1\n":                                                   "go: downloading x v1",
+	} {
+		if got := causeLine(in); got != want {
+			t.Errorf("causeLine(%q) = %q, want %q", in, got, want)
+		}
+	}
+}