quickstart: Add check that psql exists for demo branch

The demo branch uses psql to interactively guide the user through the
quickstart. The error message if psql is not on the path will eventually
be somewhate clear 'psql: command not found', but it's better to catch
this at the beginning and provide a more descriptive message.

Change-Id: Ida5cdb78825c4cb3f238268928c4fb016c3615aa
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/6682
Reviewed-by: Marcelo Altmann <[email protected]>
Tested-by: Buildkite CI

Author: Luke Osborne

quickstart/readyset_demo.sh

@@ -391,23 +391,31 @@ print_exit_message() {
   fi
 }
 
+check_psql_exists() {
+  if ! command -v psql &>/dev/null; then
+    echo -e "${RED}psql (PostgreSQL client) is not installed. Please install psql to continue.${NOCOLOR}"
+    exit 1
+  fi
+}
+
+check_mysql_exists() {
+  if ! command -v mysql &>/dev/null; then
+    echo -e "${RED}mysql (MySQL client) is not installed. Please install mysql to continue.${NOCOLOR}"
+    exit 1
+  fi
+}
+
 switch_on_mode() {
   echo -e "${BLUE}Would you like to run the demo or connect to your own db?${NOCOLOR}"
   read -rp "demo(d)/postgres(p)/mysql(m): " mode
   if [[ $mode == "p" ]]; then
-    if ! command -v psql &>/dev/null; then
-      echo -e "${RED}psql (PostgreSQL client) is not installed. Please install psql to continue.${NOCOLOR}"
-      exit 1
-    fi
-
+    check_psql_exists
     run_byo_postgres
   elif [[ $mode == "m" ]]; then
-    if ! command -v mysql &>/dev/null; then
-      echo -e "${RED}mysql (MySQL client) is not installed. Please install mysql to continue.${NOCOLOR}"
-      exit 1
-    fi
+    check_mysql_exists
     run_byo_mysql
   elif [[ $mode == "d" ]]; then
+    check_psql_exists
     run_demo
   else
     echo -e "${RED}Invalid option. Please try again.${NOCOLOR}"