Fix rendering of exceptions with generic params

Closes #145

src/Spectre.Console/Extensions/StringBuilderExtensions.cs

@@ -0,0 +1,29 @@
+using System.Globalization;
+using System.Text;
+
+namespace Spectre.Console
+{
+    internal static class StringBuilderExtensions
+    {
+        public static StringBuilder AppendWithStyle(this StringBuilder builder, Style? style, int? value)
+        {
+            return AppendWithStyle(builder, style, value?.ToString(CultureInfo.InvariantCulture));
+        }
+
+        public static StringBuilder AppendWithStyle(this StringBuilder builder, Style? style, string? value)
+        {
+            value ??= string.Empty;
+
+            if (style != null)
+            {
+                return builder.Append('[')
+                .Append(style.ToMarkup())
+                .Append(']')
+                .Append(value.EscapeMarkup())
+                .Append("[/]");
+            }
+
+            return builder.Append(value);
+        }
+    }
+}