[enh] New helper ynh_abort_if_errors (#245)

* New helper ynh_check_error

Helpers to implemente trap and set -eu.

Simply to use, you only add `ynh_check_error` in your script.

* Change helper's name

* Propagate new name in comments

data/helpers.d/system

@@ -0,0 +1,43 @@
+# Manage a fail of the script
+#
+# Print a warning to inform that the script was failed
+# Execute the ynh_clean_setup function if used in the app script
+#
+# usage of ynh_clean_setup function
+# This function provide a way to clean some residual of installation that not managed by remove script.
+# To use it, simply add in your script:
+# ynh_clean_setup () {
+#        instructions...
+# }
+# This function is optionnal.
+#
+# Usage: ynh_exit_properly is used only by the helper ynh_abort_if_errors.
+# You must not use it directly.
+ynh_exit_properly () {
+	exit_code=$?
+	if [ "$exit_code" -eq 0 ]; then
+			exit 0	# Exit without error if the script ended correctly
+	fi
+
+	trap '' EXIT	# Ignore new exit signals
+	set +eu	# Do not exit anymore if a command fail or if a variable is empty
+
+	echo -e "!!\n  $app's script has encountered an error. Its execution was cancelled.\n!!" >&2
+
+	if type -t ynh_clean_setup > /dev/null; then	# Check if the function exist in the app script.
+		ynh_clean_setup	# Call the function to do specific cleaning for the app.
+	fi
+
+	ynh_die	# Exit with error status
+}
+
+# Exit if an error occurs during the execution of the script.
+#
+# Stop immediatly the execution if an error occured or if a empty variable is used.
+# The execution of the script is derivate to ynh_exit_properly function before exit.
+#
+# Usage: ynh_abort_if_errors
+ynh_abort_if_errors () {
+	set -eu	# Exit if a command fail, and if a variable is used unset.
+	trap ynh_exit_properly EXIT	# Capturing exit signals on shell script
+}