Check parameters and add friendly exceptions.

Source/MQTTnet/Extensions/UserPropertyExtension.cs

@@ -7,12 +7,24 @@ namespace MQTTnet.Extensions
     {
         public static string GetUserProperty(this MqttApplicationMessage message, string propertyName, StringComparison comparisonType = StringComparison.OrdinalIgnoreCase)
         {
-            return message?.UserProperties?.SingleOrDefault(up => up.Name.Equals(propertyName, comparisonType))?.Value;
+            if (message == null) throw new ArgumentNullException(nameof(message));
+            if (propertyName == null) throw new ArgumentNullException(nameof(propertyName));
+
+            return message.UserProperties?.SingleOrDefault(up => up.Name.Equals(propertyName, comparisonType))?.Value;
         }
 
         public static T GetUserProperty<T>(this MqttApplicationMessage message, string propertyName, StringComparison comparisonType = StringComparison.OrdinalIgnoreCase)
         {
-            return (T) Convert.ChangeType(GetUserProperty(message, propertyName, comparisonType), typeof(T));
+            var value = GetUserProperty(message, propertyName, comparisonType);
+
+            try
+            {
+                return (T) Convert.ChangeType(value, typeof(T));
+            }
+            catch (Exception ex)
+            {
+                throw new InvalidOperationException($"Cannot convert value({value}) of UserProperty({propertyName}) to {typeof(T).FullName}.", ex);
+            }
         }
     }
 }