Name each line of a joined error under the command, test the command name as a pure function, and pin the version line's fallback

cli/cli.go

@@ -569,14 +569,20 @@ func Main(modules ...fs.FS) int {
 	if fi, err := os.Stdin.Stat(); err == nil && fi.Mode()&os.ModeCharDevice != 0 {
 		stdin = nil
 	}
-	name := filepath.Base(os.Args[0])
+	return run(commandName(os.Args[0]), modules, os.Args[1:], stdin, os.Stdout, os.Stderr)
+}
+
+// commandName is the name the command runs under: the file arg0 names, less a Windows
+// .exe, or fejkdata where arg0 names none.
+func commandName(arg0 string) string {
+	name := filepath.Base(arg0)
 	if ext := filepath.Ext(name); strings.EqualFold(ext, ".exe") {
 		name = strings.TrimSuffix(name, ext)
 	}
 	if name == "." || name == "" {
-		name = "fejkdata"
+		return "fejkdata"
 	}
-	return run(name, modules, os.Args[1:], stdin, os.Stdout, os.Stderr)
+	return name
 }
 
 // run is Main over the given streams, run as the command name. A nil stdin is a terminal,
@@ -657,9 +663,10 @@ func misuse(stderr io.Writer, name string, err error) int {
 	return 2
 }
 
-// report writes err under the command name, in place of the library's own.
+// report writes err under the command name, in place of the library's own on each line.
 func report(stderr io.Writer, name string, err error) {
-	fmt.Fprintf(stderr, "%s: %s\n", name, strings.TrimPrefix(err.Error(), "fejkdata: "))
+	msg := strings.ReplaceAll(strings.TrimPrefix(err.Error(), "fejkdata: "), "\nfejkdata: ", "\n"+name+": ")
+	fmt.Fprintf(stderr, "%s: %s\n", name, msg)
 }
 
 // fejkdataModule is the path of the module holding the library; the root package's path is

cli/cli_test.go

@@ -456,6 +456,7 @@ func TestVersionLineNamesFejkdatasVersion(t *testing.T) {
 		{&debug.BuildInfo{Main: mycli, Deps: []*debug.Module{fejkdata("v0.1.0", nil)}}, "mycli v9.0.0, fejkdata v0.1.0"},
 		{&debug.BuildInfo{Main: mycli, Deps: []*debug.Module{fejkdata("v0.1.0", &debug.Module{Path: "../fejkdata"})}}, "mycli v9.0.0, fejkdata devel"},
 		{&debug.BuildInfo{Main: mycli, Deps: []*debug.Module{fejkdata("v0.1.0", &debug.Module{Path: "example.com/fork", Version: "v0.1.1"})}}, "mycli v9.0.0, fejkdata v0.1.1"},
+		{&debug.BuildInfo{Main: mycli}, "mycli v9.0.0, fejkdata devel"},
 		{nil, "fejkdata devel"},
 	} {
 		if got := versionLine("mycli", c.info); got != c.want {
@@ -464,6 +465,20 @@ func TestVersionLineNamesFejkdatasVersion(t *testing.T) {
 	}
 }
 
+func TestCommandNameIsTheFileItRunsFrom(t *testing.T) {
+	for arg0, want := range map[string]string{
+		"/usr/local/bin/mycli": "mycli",
+		"mycli.EXE":            "mycli",
+		"mycli.exe":            "mycli",
+		"my.cli":               "my.cli",
+		"":                     "fejkdata",
+	} {
+		if got := commandName(arg0); got != want {
+			t.Errorf("commandName(%q) = %q, want %q", arg0, got, want)
+		}
+	}
+}
+
 func TestRunNamesItselfByTheCommandName(t *testing.T) {
 	var out, errb bytes.Buffer
 	if code := run("mycli", shipped, []string{"--nope"}, nil, &out, &errb); code != 2 || errb.String() != "mycli: unknown flag --nope\ntry 'mycli --help'\n" {
@@ -474,7 +489,11 @@ func TestRunNamesItselfByTheCommandName(t *testing.T) {
 		t.Errorf("mycli with nothing piped = %d, %q, want a pipe into mycli", code, errb.String())
 	}
 	errb.Reset()
-	if code := run("mycli", shipped, []string{"-d", "/nope"}, strings.NewReader("{/sv_SE.person}"), &out, &errb); code != 1 || !strings.HasPrefix(errb.String(), "mycli: ") {
+	if code := run("mycli", []fs.FS{nil, nil}, nil, strings.NewReader("x"), &out, &errb); code != 1 || strings.Count(errb.String(), "mycli: ") != 2 || strings.Contains(errb.String(), "fejkdata") {
+		t.Errorf("mycli over two nil modules = %d, %q, want each line under mycli", code, errb.String())
+	}
+	errb.Reset()
+	if code := run("mycli", shipped, []string{"-d", "/nope"}, strings.NewReader("{/sv_SE.person}"), &out, &errb); code != 1 || !strings.HasPrefix(errb.String(), "mycli: ") || strings.Contains(errb.String(), "fejkdata") {
 		t.Errorf("mycli -d /nope = %d, %q, want a load error under mycli", code, errb.String())
 	}
 	if code := run("mycli", shipped, []string{"--help"}, nil, &out, &errb); code != 0 || !strings.HasPrefix(out.String(), "Usage: <template> | mycli [flags]\n") || strings.Contains(out.String(), "fejkdata") {