Remove public constructor for MySqlConversionException.

This type should not be constructible by consumers.

Author: bgrainger

src/MySqlConnector/MySqlConversionException.cs

@@ -17,7 +17,7 @@ public class MySqlConversionException : Exception
 		/// Initializes a new instance of <see cref="MySqlConversionException"/>.
 		/// </summary>
 		/// <param name="message">The exception message.</param>
-		public MySqlConversionException(string message)
+		internal MySqlConversionException(string message)
 			: base(message)
 		{
 		}