Improved error messages for plot addition. (#3600)

* improved error messages for plot addition.

* Use same language for theme addition errors.

Author: clauswilke

R/plot-construction.r

@@ -88,7 +88,7 @@ ggplot_add <- function(object, plot, object_name) {
 }
 #' @export
 ggplot_add.default <- function(object, plot, object_name) {
-  stop("Don't know how to add ", object_name, " to a plot", call. = FALSE)
+  stop("Can't add `", object_name, "` to a ggplot object.", call. = FALSE)
 }
 #' @export
 ggplot_add.NULL <- function(object, plot, object_name) {
@@ -102,8 +102,9 @@ ggplot_add.data.frame <- function(object, plot, object_name) {
 #' @export
 ggplot_add.function <- function(object, plot, object_name) {
   stop(
-    "Don't know how to add ", object_name, " to a plot. Did you mean ",
-    object_name, "()?", call. = FALSE
+    "Can't add `", object_name, "` to a ggplot object.\n",
+    "Did you forget to add parentheses, as in `",
+    object_name, "()`?", call. = FALSE
   )
 }
 #' @export

R/theme.r

@@ -436,7 +436,7 @@ plot_theme <- function(x, default = theme_get()) {
 #' @keywords internal
 add_theme <- function(t1, t2, t2name) {
   if (!is.theme(t2)) {
-    stop("Don't know how to add ", t2name, " to a theme object",
+    stop("Can't add `", t2name, "` to a theme object.",
       call. = FALSE)
   }