set errorbar width and thickness

Set an error bar's width and thickness.

errorbarT(H,W,T) adjusts the width of error bars with handle H.
The input W is given in the units of the current x-axis.
The input T is the 'LineWidth' of the errorbar

use it like this:

h = errorbar(....);

errorbar_tick(h, .5, 2);

Based on code by Arnaud Laurent.