SQL Plugin: The `preload` parameter has a potential `migration` requirement.

[Goodjooy]

Problem

When setting up the sql plugin, the ‘preload’ parameter is only used for migrations and assumes that the migration is given

plugins-workspace/plugins/sql/src/lib.rs

Lines 145 to 153 in 7e1c17a

	for db in config.preload {
	let pool = DbPool::connect(&db, app).await?;
	if let Some(migrations) = self.migrations.as_mut().unwrap().remove(&db) {
	let migrator = Migrator::new(migrations).await?;
	pool.migrate(&migrator).await?;
	}
	lock.insert(db, pool);

This does not seem to match its semantics.

If I use the SQL plugin and give a preload parameter but don't provide any migrations, the programme reports the following panic message

This is very unintuitive, and although preload is currently only used by migration, it would be better to report an error like no migration providing.

Improvements

I have the following ideas to fix the current problem

1. rename preload to something else related to migration, such as migration-preload.
2. preload is not only used for migrations, but also for opening connections and managing them
3. provide a more user friendly panic message