*: Move die() into .local/bin/helpers.sh

The function `die` was redundantly implemented in various files.

Move the function into .local/bin/helpers.sh and source that where
previously implemented.

Also prepend the program's name to the message and always terminate the
message with a newline. The newline was previously needed for a small
but unnecessary hack that prevented the need of the `[ -z "$1" ]` test.

.local/bin/helpers.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+#
+# A collection of useful shell functions. This file should be sourced, rather
+# than executed.
+
+
+# Print error message, prepended by the programs name, and then exit
+#
+# Usage: die [<MESSAGE>] [<EXIT_CODE>]
+die() {
+	[ -z "$1" ] || >&2 printf "%s: %s\n" "$0" "$1"
+	exit ${2:-1}
+}