Update setup script.

setup.sh

@@ -1,7 +1,29 @@
 #!/usr/bin/env bash
-git submodule update --init
+set -Eeuo pipefail
+
+# $1: Error message
+# $2: Error code
+function fatal() {
+  local msg=${1:-"Unspecified error"}
+  echo "[Error:$LINENO] Could not install dotfiles: $msg"
+  exit ${2:-1}
+}
+trap 'fatal "Trapped an unknown error" $?' ERR
+
+if ! git rev-parse --is-inside-work-tree >/dev/null 2>&1; then
+  fatal "Run setup.sh from the root directory of the dotfiles repository." 2
+fi
+git submodule update --init --recursive
+
 # Use alias for installing packages so we don't maintain several dependency lists.
+if [[ ! -f .bash_aliases ]]; then
+  fatal "Failed to locate .bash_aliases in path: $(pwd)" 3
+fi
 . .bash_aliases
+
+if [[ -z ${DOT_PACKAGES:-} ]]; then
+	fatal "DOT_PACKAGES variable not found after sourcing $(pwd)/.bash_aliases" 4
+fi
 sudo apt update -y && sudo apt install -y $DOT_PACKAGES
 go install github.com/arl/gitmux@latest