Expose loot-condition-interpreter error codes to callers

Make ConditionSyntaxError extend from system_error instead of directly from runtime_error, so that callers can use the exception's code to distinguish between different causes.

include/loot/exception/condition_syntax_error.h

@@ -32,9 +32,9 @@ namespace loot {
  * @brief An exception class thrown if invalid syntax is encountered when
  *        parsing a metadata condition.
  */
-class ConditionSyntaxError : public std::runtime_error {
+class ConditionSyntaxError : public std::system_error {
 public:
-  using std::runtime_error::runtime_error;
+  using std::system_error::system_error;
 };
 }
 

include/loot/exception/error_categories.h

@@ -45,6 +45,14 @@ LOOT_API const std::error_category& esplugin_category();
  *          derived from std::error_category.
  */
 LOOT_API const std::error_category& libloadorder_category();
+
+/**
+ * @brief Get the error category that can be used to identify system_error
+ *        exceptions that are due to loot condition interpreter errors.
+ * @returns A reference to the static object of unspecified runtime type,
+ *          derived from std::error_category.
+ */
+LOOT_API const std::error_category& loot_condition_interpreter_category();
 }
 
 #endif