informative RED runtime error message when type error detected

internal/color/color.go

@@ -0,0 +1,86 @@
+package color
+
+import "runtime"
+
+const (
+	reset  = "\033[0m"
+	red    = "\033[31m"
+	green  = "\033[32m"
+	yellow = "\033[33m"
+	blue   = "\033[34m"
+	purple = "\033[35m"
+	cyan   = "\033[36m"
+	gray   = "\033[37m"
+	white  = "\033[97m"
+)
+
+var (
+	isWindows bool
+)
+
+func init() {
+	isWindows = runtime.GOOS == "windows"
+}
+
+func Reset() string {
+	if isWindows {
+		return ""
+	}
+	return reset
+}
+
+func Red() string {
+	if isWindows {
+		return ""
+	}
+	return red
+}
+
+func Green() string {
+	if isWindows {
+		return ""
+	}
+	return green
+}
+
+func Yellow() string {
+	if isWindows {
+		return ""
+	}
+	return yellow
+}
+
+func Blue() string {
+	if isWindows {
+		return ""
+	}
+	return blue
+}
+
+func Purple() string {
+	if isWindows {
+		return ""
+	}
+	return purple
+}
+
+func Cyan() string {
+	if isWindows {
+		return ""
+	}
+	return cyan
+}
+
+func Gray() string {
+	if isWindows {
+		return ""
+	}
+	return gray
+}
+
+func White() string {
+	if isWindows {
+		return ""
+	}
+	return white
+}

internal/handler/handler.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"runtime"
 
+	"github.com/lainio/err2/internal/color"
 	"github.com/lainio/err2/internal/debug"
 	fmtstore "github.com/lainio/err2/internal/formatter"
 	"github.com/lainio/err2/internal/str"
@@ -340,8 +341,13 @@ func processArg(info *Info, i int, a ...any) {
 		info.NilHandler = NilNoop
 	default:
 		// we don't panic here because we can already be in recovery, but lets
-		// try to show an error message at least.
-		fmt.Fprintln(os.Stderr, "fatal error: err2.Handle: unsupported type")
+		// try to show an RED error message at least.
+		const msg = `err2 fatal error:  
+---
+unsupported handler function type: err2.Handle/Catch:
+see 'err2/scripts/README.md' and run auto-migration scripts for your repo
+---`
+		fmt.Fprintln(os.Stderr, color.Red()+msg+color.Reset())
 	}
 }