Error bars for barplot only in one direction, but for the last one in the opposite direction

Question

This very useful post shows how to display error bars in one direction only with ggplot. To do this, the geom_errorbar function is modified as follows (as proposed by Sean Hughes):

geom_uperrorbar <- function(mapping = NULL, data = NULL,
   stat = "identity", position = "identity",
   ...,
   na.rm = FALSE,
   show.legend = NA,
   inherit.aes = TRUE) {
   layer(
      data = data,
      mapping = mapping,
      stat = stat,
      geom = GeomUperrorbar,
      position = position,
      show.legend = show.legend,
      inherit.aes = inherit.aes,
      params = list(
         na.rm = na.rm,
         ...
      )
   )
}

GeomUperrorbar <- ggproto("GeomUperrorbar", Geom,
   default_aes = aes(colour = "black", size = 0.5, linetype = 1, width = 0.5,
      alpha = NA),

   draw_key = draw_key_path,
required_aes = c("x", "y", "ymax"),

   setup_data = function(data, params) {
      data$width <- data$width %||%
         params$width %||% (resolution(data$x, FALSE) * 0.9)

      transform(data,
         xmin = x - width / 2, xmax = x + width / 2, width = NULL
      )
   },draw_panel = function(data, panel_scales, coord, width = NULL) {
      GeomPath$draw_panel(data.frame(
         x = as.vector(rbind(data$xmin, data$xmax, NA, data$x,   data$x)),
         y = as.vector(rbind(data$ymax, data$ymax, NA, data$ymax, data$y)),
         colour = rep(data$colour, each = 5),
         alpha = rep(data$alpha, each = 5),
         size = rep(data$size, each = 5),
         linetype = rep(data$linetype, each = 5),
         group = rep(1:(nrow(data)), each = 5),
         stringsAsFactors = FALSE,
         row.names = 1:(nrow(data) * 5)
      ), panel_scales, coord)
   }
)


"%||%" <- function(a, b) {
   if (!is.null(a)) a else b
} 

But now I have the case that the last bar is negative. Thus, the error bar at the last bar must head in the other direction. Does anyone have an idea how this could work?

Here is a reproducible example:

df <- data.frame(trt = factor(c(1, 1, 2, 2)), resp = c(1, 5, 3, -2),
                 group = factor(c(1, 2, 1, 2)), se = c(0.1, 0.3, 0.3, 0.2))
df2 <- df[c(1,3), ]

limits <- aes(ymax = resp + se, ymin = resp - se)
dodge <- position_dodge(width = 0.9)

p <- ggplot(df, aes(fill = group, y = resp, x = trt))
p + geom_bar(position = dodge, stat = "identity") +
  geom_errorbar(limits, position = dodge, width = 0.25)