config/functions: add die() function

die is meant to be a more flexible exit, printing a message to go with
the exit call. It works like:

die -- same as "exit 1"
die "It just broke." -- echo statement, exit value 1
die "Tried to do the thing." "2" -- echo statement, exit value 2

Exit codes other than 1 require a message to go with it.

it would replace existing code that looks like:

echo "ERROR: Everything went wrong. Sorry!"
exit 1

or be more helpful in picking up the pieces when something unexpected
happens then:

do_something || exit 1

Signed-off-by: Ian Leonard <antonlacon@gmail.com>

config/functions

@@ -1,4 +1,12 @@
 ### FUNCTION HELPERS ###
+# die (message, code) abort with optional message and code
+die() {
+  if [ -n "$1" ]; then
+    echo -e "$1" >&2
+  fi
+  exit "${2:-1}"
+}
+
 # return 0 if $2 in space-separated list $1, otherwise return 1
 listcontains() {
   if [ -n "$1" -a -n "$2" ]; then