Change to InvalidOperationException.

This matches the (future) behaviour of Microsoft.Data.Sqlite: aspnet/Microsoft.Data.Sqlite@8e4db99#diff-0561eef0368965f9acff5b0472c65fc6

Author: bgrainger

src/MySqlConnector/MySql.Data.MySqlClient/MySqlCommand.cs

@@ -92,7 +92,7 @@ public override void Prepare()
 			set
 			{
 				if (m_connection?.HasActiveReader ?? false)
-					throw new MySqlException("Cannot set MySqlCommand.Connection when there is an open DataReader for this command; it must be closed first.");
+					throw new InvalidOperationException("Cannot set MySqlCommand.Connection when there is an open DataReader for this command; it must be closed first.");
 				m_connection = value;
 			}
 		}