Unclear error when using add_index with column which already has an index

Consider the following migration:

~~~crystal
class CreateTags::V20190623184241 < Avram::Migrator::Migration::V1
def migrate
create :tags do
add name : String, unique: true
end

add_index :name, unique: true
end

def rollback
drop :tags
end
end
~~~

The `unique: true` parameter to the `add` method automatically creates a unique index for that column. I didn't realize this initially, and adding the `add_index` statement below caused the migration to fail with a really unclear error:

https://gist.github.com/taylorthurlow/98bc7018ea9254ceb7e476ca87c368a3

I solved the issue by just removing the redundant `add_index` call. In the future it might be a better idea to fail with a descriptive error message.

Start by tracing Avram's migration add_index path and the unique: true handling shown in the example; inspect the failing migration behavior from the linked gist. Done means attempting a duplicate index produces a descriptive error explaining that the column already has an index, with coverage for this migration case.

Written by the indexing model from the issue text.