Check that db/ exists before running db:schema:load
Check that db/ exists before running db:schema:load

This is essentially the same issue as #156. An easy fix and useful
feedback for users who created a Rails app without ActiveRecord and
will not have any `db/` directory.

The feedback is:

```
$ bin/rails db:schema:dump
The db/ directory doesn't exist, please create it.
```

Compare to what happened before this commit:

```
$ bin/rails db:schema:dump
rails aborted!
Errno:36383dc93ad24487d6cae4b2a13a93f4e77076cf@ rb_sysopen - .../sandbox/appname/db/schema.rb
.../oss/sequel-rails/lib/sequel_rails/railties/database.rake:26:in `initialize'
.../oss/sequel-rails/lib/sequel_rails/railties/database.rake:26:in `open'
.../oss/sequel-rails/lib/sequel_rails/railties/database.rake:26:in `block (3 levels) in <top (required)>'
.../sandbox/appname/bin/rails:9:in `require'
.../sandbox/appname/bin/rails:9:in `<top (required)>'
.../sandbox/appname/bin/spring:15:in `<top (required)>'
bin/rails:3:in `load'
bin/rails:3:in `<main>'
Tasks: TOP => db:schema:dump
(See full trace by running task with --trace)
```

TalentBox/sequel-rails,TalentBox/sequel-rails