Rework handling of esplugin errors

The error scenario represented by PluginNotLoadedError is distinguished from all other esplugin errors by LOOT, so it's worth handling separately.

cpp/include/loot/api.h

@@ -35,7 +35,7 @@
 #include "loot/enum/game_type.h"
 #include "loot/enum/log_level.h"
 #include "loot/exception/cyclic_interaction_error.h"
-#include "loot/exception/error_categories.h"
+#include "loot/exception/plugin_not_loaded_error.h"
 #include "loot/exception/undefined_group_error.h"
 #include "loot/game_interface.h"
 #include "loot/loot_version.h"

cpp/include/loot/exception/plugin_not_loaded_error.h

@@ -22,21 +22,20 @@
     <https://www.gnu.org/licenses/>.
     */
 
-#ifndef LOOT_ERROR_CATEGORIES
-#define LOOT_ERROR_CATEGORIES
+#ifndef LOOT_EXCEPTION_PLUGIN_NOT_LOADED_ERROR
+#define LOOT_EXCEPTION_PLUGIN_NOT_LOADED_ERROR
 
-#include <system_error>
-
-#include "loot/api_decorator.h"
+#include <stdexcept>
 
 namespace loot {
 /**
- * @brief Get the error category that can be used to identify system_error
- *        exceptions that are due to esplugin errors.
- * @returns A reference to the static object of unspecified runtime type,
- *          derived from std::error_category.
+ * @brief An exception class thrown if a plugin that must be loaded hasn't been
+ *        loaded.
  */
-LOOT_API const std::error_category& esplugin_category();
+class PluginNotLoadedError : public std::runtime_error {
+public:
+  using std::runtime_error::runtime_error;
+};
 }
 
 #endif