Make the wine-not-found error actionable

FindWine's error used to just say wine64/wine weren't on PATH, with no
next step - surface the exact install fix ("install the wine
package") instead of leaving the reader to figure that out
themselves.

internal/wineenv/wine.go

@@ -14,5 +14,5 @@ func FindWine() (string, error) {
 	if p, err := exec.LookPath("wine"); err == nil {
 		return p, nil
 	}
-	return "", fmt.Errorf("neither wine64 nor wine found in PATH")
+	return "", fmt.Errorf("neither wine64 nor wine found in PATH (install the wine package)")
 }

internal/wineenv/wine_test.go

@@ -0,0 +1,60 @@
+package wineenv
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func fakeBinary(t *testing.T, name string) {
+	t.Helper()
+	bin := t.TempDir()
+	path := filepath.Join(bin, name)
+	if err := os.WriteFile(path, []byte("#!/bin/sh\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", bin)
+}
+
+func TestFindWinePrefersWine64(t *testing.T) {
+	bin := t.TempDir()
+	for _, name := range []string{"wine64", "wine"} {
+		if err := os.WriteFile(filepath.Join(bin, name), []byte("#!/bin/sh\n"), 0o755); err != nil {
+			t.Fatal(err)
+		}
+	}
+	t.Setenv("PATH", bin)
+
+	got, err := FindWine()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if filepath.Base(got) != "wine64" {
+		t.Errorf("FindWine() = %q, want wine64 to be preferred over wine", got)
+	}
+}
+
+func TestFindWineFallsBackToWine(t *testing.T) {
+	fakeBinary(t, "wine")
+
+	got, err := FindWine()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if filepath.Base(got) != "wine" {
+		t.Errorf("FindWine() = %q, want wine", got)
+	}
+}
+
+func TestFindWineErrorIsActionable(t *testing.T) {
+	t.Setenv("PATH", t.TempDir()) // empty dir, neither binary present
+
+	_, err := FindWine()
+	if err == nil {
+		t.Fatal("expected an error when neither wine64 nor wine is on PATH")
+	}
+	if !strings.Contains(err.Error(), "install") {
+		t.Errorf("FindWine() error = %q, want it to say what to install (matching msiextract/cabextract's error style)", err)
+	}
+}