chore: update documentation

plugins/sql/src/error.rs

@@ -4,16 +4,30 @@
 
 use serde::{Serialize, Serializer};
 
+/// Errors that can happen while connecting to a database, running migrations
+/// or executing a query.
+///
+/// Serializes to its [`std::fmt::Display`] representation, which is what the
+/// frontend receives when a command fails.
 #[derive(Debug, thiserror::Error)]
 pub enum Error {
+    /// An error reported by [`sqlx`], such as a failed connection or a query the database rejected.
     #[error(transparent)]
     Sql(#[from] sqlx::Error),
+    /// A migration registered with [`crate::Builder::add_migrations`] could not be resolved or applied.
     #[error(transparent)]
     Migration(#[from] sqlx::migrate::MigrateError),
+    /// The connection string is missing its `scheme:` prefix, or the scheme does not
+    /// match any of the enabled database drivers. Contains the offending connection string.
     #[error("invalid connection url: {0}")]
     InvalidDbUrl(String),
+    /// The requested database has not been connected to with the `load` command
+    /// and is not listed in the plugin's `preload` configuration.
+    /// Contains the connection string of the database.
     #[error("database {0} not loaded")]
     DatabaseNotLoaded(String),
+    /// A value selected from the database has a SQL type that cannot be converted
+    /// to JSON. Contains the name of that SQL type.
     #[error("unsupported datatype: {0}")]
     UnsupportedDatatype(String),
 }