Add UndefinedGroupError for better sorting exception handling

include/loot/api.h

@@ -37,6 +37,7 @@
 #include "loot/exception/error_categories.h"
 #include "loot/exception/file_access_error.h"
 #include "loot/exception/git_state_error.h"
+#include "loot/exception/undefined_group_error.h"
 #include "loot/game_interface.h"
 #include "loot/loot_version.h"
 

include/loot/exception/undefined_group_error.h

@@ -0,0 +1,58 @@
+/*  LOOT
+
+    A load order optimisation tool for Oblivion, Skyrim, Fallout 3 and
+    Fallout: New Vegas.
+
+    Copyright (C) 2012-2016    WrinklyNinja
+
+    This file is part of LOOT.
+
+    LOOT is free software: you can redistribute
+    it and/or modify it under the terms of the GNU General Public License
+    as published by the Free Software Foundation, either version 3 of
+    the License, or (at your option) any later version.
+
+    LOOT is distributed in the hope that it will
+    be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with LOOT.  If not, see
+    <https://www.gnu.org/licenses/>.
+    */
+
+#ifndef LOOT_EXCEPTION_UNDEFINED_GROUP_ERROR
+#define LOOT_EXCEPTION_UNDEFINED_GROUP_ERROR
+
+#include <stdexcept>
+
+namespace loot {
+/**
+ * @brief An exception class thrown if group is referenced but is undefined.
+ */
+class UndefinedGroupError : public std::runtime_error {
+public:
+  /**
+   * @brief Construct an exception detailing a plugin graph cycle.
+   * @param firstPlugin A plugin in the cycle.
+   * @param lastPlugin Another plugin in the cycle.
+   * @param backCycle A string describing the path from lastPlugin to
+   *                  firstPlugin.
+   */
+  UndefinedGroupError(const std::string& groupName) :
+      std::runtime_error("The group \"" + groupName + "\" does not exist"),
+      groupName_(groupName) {}
+
+  /**
+   * Get the name of the undefined group.
+   * @return A group name.
+   */
+  std::string getGroupName() { return groupName_; }
+
+private:
+  const std::string groupName_;
+};
+}
+
+#endif