Add 'die' function

Candidate replacement of multiple instances of the same error
handling sequence.

imgbb

@@ -62,6 +62,15 @@ print_usage() {
   echo "  -h, --help             Show this help message."
 }
 
+die() {
+  declare msg
+
+  msg="$*"
+  notify_cmd -u critical "ImgBB Upload Error" "$msg" &>/dev/null
+  echo "$msg" >&2
+  exit 1
+}
+
 # Parse command-line options.
 while [[ $# -gt 0 ]]; do
   key="$1"