I have encountered the error message "% PLOT: Unknown property: ERRORBAR_THICK", when I was trying to control the size of my errorbar thickness. I was seaching the forum for answers but found none. Now I have discovered that ERRORBAR_LINESTYLE and ERRORBAR_THICK are new features in 8.4, and I only have 8.3. So here is the information if anyone at a future point is searching the forum with the same problem as I had.