Let(
    [
        e = MBS( "IsError" ) ;
        r = If(
            e ;
            Let(
                [
                    errorCode = MBS( "FM.ExecuteSQL.LastError" ) ;
                    errorText = MBS( "FM.ExecuteSQL.LastErrorMessage" ) ;
                    sql = MBS( "FM.ExecuteSQL.LastSQL" ) ;
                    path = "/Users/cs/Desktop/sql.log" ;
                    r = MBS(
                        "Text.AppendTextFile" ;
                        "Error " &
                        errorCode &
                        ": " &
                        errorText &
                        " while running " &
                        sql &
                        ¶ &
                        ¶ ;
                        path
                    )
                ] ;
                ""
            )
        )
    ] ;
    ""
)

// to be used after a MBS() call with SQL to log the error.
Custom function to log the last SQL error to a log file:

Let([
    sql = "SELECT * FROM Test";
    r = MBS("FM.ExecuteFileSQL"; ""; sql);
    errorCode = MBS( "FM.ExecuteSQL.LastError" );
    lastSQL = MBS("FM.ExecuteSQL.LastSQL")
]; errorCode & ": " & lastSQL)
Query last SQL after causing an error:
Example result: 8309: SELECT * FROM Test

Let([
    sql = "INSERT INTO Test (FirstName, Age) VALUES (?,?)";
    r = MBS("FM.ExecuteFileSQL"; ""; sql; 9; 13; "Joe"; 23);

    errorCode = MBS( "FM.ExecuteSQL.LastError" );
    errorMessage = MBS("FM.ExecuteSQL.LastErrorMessage");
    lastSQL = MBS("FM.ExecuteSQL.LastSQL");
    params = MBS("FM.ExecuteSQL.LastParameters");

    json = JSONSetElement ( "{}" ;
        ["Parameters"; params; JSONArray];
        ["ErrorCode"; errorCode; JSONNumber];
        ["SQL"; lastSQL; JSONString];
        ["ErrorMessage"; errorMessage; JSONString])
]; JSONFormatElements ( json ))
Build a JSON after causing an error:
Example result:
{ "ErrorCode" : 8309, "ErrorMessage" : "ERROR: FQL0002/(1:12): The table named \"Test\" does not exist.", "Parameters" : [ "Joe", 23 ], "SQL" : "INSERT INTO Test (FirstName, Age) VALUES (?,?)" }