Fix: check and propagate errors in AsyncBackend

Closes #35

R/AsyncBackend.R

@@ -263,8 +263,17 @@ AsyncBackend <- R6::R6Class("AsyncBackend",
 
         # Set the output based on the session read.
         .set_output = function() {
-            # Store the relevant results from the output.
-            private$.output <- private$.cluster$read()$result
+            # Get all session output.
+            output <- private$.cluster$read()
+
+            # If an error ocurred in the session.
+            if (!is.null(output$error)) {
+                # Throw error in the main session.
+                Exception$async_task_error(output$error)
+            }
+
+            # Otherwise, store the relevant results from the output.
+            private$.output <- output$result
         },
 
         # Get the current task state (i.e., what is happening in the session).

R/Exception.R

@@ -18,6 +18,7 @@
 #'   \item{\code{Exception$async_task_not_started()}}{Exception for reading results while an asynchronous task has not yet started.}
 #'   \item{\code{Exception$async_task_running()}}{Exception for reading results while an asynchronous task is running.}
 #'   \item{\code{Exception$async_task_completed()}}{Exception for reading results while a completed asynchronous task has unread results.}
+#'   \item{\code{Exception$async_task_error(error)}}{Exception for errors while running an asynchronous task.}
 #'   \item{\code{Exception$temporary_file_creation_failed()}}{Exception for reading results while an asynchronous task is running.}
 #'   \item{\code{Exception$type_not_assignable(actual, expected)}}{Exception for when providing incorrect object types.}
 #'   \item{\code{Exception$unknown_package_option(option)}}{Exception for when requesting unknown package options.}
@@ -92,6 +93,12 @@ Exception$async_task_completed <- function() {
     stop("A task is completed with unread results.", call. = FALSE)
 }
 
+# Exception for errors in the session while running an asynchronous task.
+Exception$async_task_error <- function(error) {
+    # Throw the error.
+    stop(error)
+}
+
 # Exception for reading results while an asynchronous task is running.
 Exception$temporary_file_creation_failed <- function(path) {
     # Construct exception message.