db/Plugin: migrate from class Error to C++ exceptions

src/db/DatabasePlugin.hxx

@@ -27,7 +27,6 @@
 #define MPD_DATABASE_PLUGIN_HXX
 
 struct ConfigBlock;
-class Error;
 class EventLoop;
 class DatabaseListener;
 class Database;
@@ -45,10 +44,11 @@ struct DatabasePlugin {
 
 	/**
 	 * Allocates and configures a database.
+	 *
+	 * Throws #std::runtime_error on error.
 	 */
 	Database *(*create)(EventLoop &loop, DatabaseListener &listener,
-			    const ConfigBlock &block,
-			    Error &error);
+			    const ConfigBlock &block);
 
 	constexpr bool RequireStorage() const {
 		return flags & FLAG_REQUIRE_STORAGE;