Raise an error when index name is invalid

Prevents this kind of thing:

```ruby
change_table :foo do |m|
  m.add_index [:bar], name: 'bar'
end
```

```sql
create index `{:name=>"bar"}` on `lhmn_foo` (`bar`)
```

@arthurnn @camilo 
cc @sroysen @SingerWang