Schema API should report errors in the schema type definition

If you have an error in the type name in a schema definition, Drupal doesn't detect it. Instead, at least with MySQL, you get a SQL error, and MySQL's syntax error reporting is less than ideal.

For example, I entered 'type' => 'integer' in one of my column definitions in an install file. (I've been doing a lot of Propel, which uses integer instead of int.) Because of the way the error was reported and formatted out of MySQL, I spent a lot of time barking up the wrong tree.

I believe that for both MySQL and PostgresSQL, Drupal just does a quick lookup in a map table, in the corresponding _db_process_field function. It doesn't check whether the type is actually set in the map or whether the resulting type string is non-empty. However, I'm not sure whether the correct solution is to simply call watchdog here to emit a warning or error message, or to do it at a higher level.