Make the container type for fetch customizable

lib/Database/PostgreSQL/Opium/Error.hs

@@ -17,6 +17,7 @@ data Error
   | ErrorInvalidOid Text Oid
   | ErrorUnexpectedNull ErrorPosition
   | ErrorInvalidField ErrorPosition Oid ByteString String
+  | ErrorNotExactlyOneRow
   deriving (Eq, Show)
 
 instance Exception Error where

lib/Database/PostgreSQL/Opium/FromRow.hs

@@ -14,6 +14,7 @@ module Database.PostgreSQL.Opium.FromRow
   -- * FromRow
   ( FromRow (..)
   -- * Internal
+  , ColumnTable
   , toListColumnTable
   ) where