Improve the error message for a bad DB value in bin/sandbox

Author: elia

lib/solidus_dev_support/templates/extension/bin/sandbox.tt

@@ -9,15 +9,16 @@ postgres|postgresql)
 mysql)
   RAILSDB="mysql"
   ;;
-sqlite)
+sqlite3|sqlite)
   RAILSDB="sqlite3"
   ;;
 '')
   echo "~~> Use 'export DB=[postgres|mysql|sqlite]' to control the DB adapter"
   RAILSDB="sqlite3"
   ;;
 *)
-  echo "Invalid DB specified: $DB"
+  echo "Invalid value specified for the Solidus sandbox: DB=\"$DB\"."
+  echo "Please use 'postgres', 'mysql', or 'sqlite' instead."
   exit 1
   ;;
 esac