Fix display of errors.

In some settings it might occur that the output of stdout and error overlap
which produces output like the following.

    ERROR:  syntaxTime: 1.118 ms
    ar "table_name"
    LINE 1: select * table_name;
                     ^

After applying this patch vim-simpledb writes the error to a separate file
which gets then appended to the output so that we get a nicer error message
like the following

    Timing is on.
    Time: 1.421 ms
    ERROR:  syntax error at or near "table_name"
    LINE 1: select * table_name;
                     ^