Raise useful exception when db/migrate directory is absent

[olivierlacan]

Currently when you run bin/rails db:migrate on a fresh rails app
created with: rails new appname --skip-active-record you don't get a
db/migrate directory which results in this exception:

rails aborted!
Errno::ENOENT: No such file or directory @ dir_initialize - .../db/migrate
.../oss/sequel-rails/lib/sequel_rails/migrations.rb:9:in `migrate'
.../oss/sequel-rails/lib/sequel_rails/railties/database.rake:166:in `block (2 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:migrate
(See full trace by running task with --trace)

With this fix SequelRails will catch this issue before it's too late and
gets to Sequel's migration logic (which assumes db/migrate exists). This
is the error produced now:

rails aborted!
The db/migrate directory doesn't exist, you need to create it.
.../oss/sequel-rails/lib/sequel_rails/migrations.rb:14:in `migrate'
.../oss/sequel-rails/lib/sequel_rails/railties/database.rake:166:in `block (2 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:migrate
(See full trace by running task with --trace)

After following the instruction from the error mkdir -p db/migrate,
you'll then get this much friendlier error from Sequel::Migrator itself:

$ bin/rails db:migrate
rails aborted!
Sequel::Migrator::Error: No target version available, probably because no migration files found or filenames don't follow the migration filename convention
.../oss/sequel-rails/lib/sequel_rails/migrations.rb:11:in `migrate'
.../oss/sequel-rails/lib/sequel_rails/railties/database.rake:166:in `block (2 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:migrate
(See full trace by running task with --trace)

[JonathanTron]

@olivierlacan This is a great improvement thanks!