Make UndefinedGroupError::GetGroupName() const

Also move the constructor and function definitions out of the header
where they're declared.

CMakeLists.txt

@@ -193,6 +193,7 @@ set(LIBLOOT_SRC_API_CPP_FILES
     "${CMAKE_SOURCE_DIR}/src/api/sorting/plugin_sort.cpp"
     "${CMAKE_SOURCE_DIR}/src/api/sorting/plugin_graph.cpp"
     "${CMAKE_SOURCE_DIR}/src/api/sorting/plugin_sorting_data.cpp"
+    "${CMAKE_SOURCE_DIR}/src/api/sorting/undefined_group_error.cpp"
     "${CMAKE_SOURCE_DIR}/src/api/helpers/crc.cpp"
     "${CMAKE_SOURCE_DIR}/src/api/helpers/text.cpp"
     "${CMAKE_SOURCE_DIR}/src/api/vertex.cpp")

include/loot/exception/undefined_group_error.h

@@ -39,15 +39,13 @@ public:
    * @brief Construct an exception for an undefined group.
    * @param groupName The name of the group that is undefined.
    */
-  LOOT_API UndefinedGroupError(const std::string& groupName) :
-      std::runtime_error("The group \"" + groupName + "\" does not exist"),
-      groupName_(groupName) {}
+  LOOT_API UndefinedGroupError(const std::string& groupName);
 
   /**
    * Get the name of the undefined group.
    * @return A group name.
    */
-  LOOT_API std::string GetGroupName() { return groupName_; }
+  LOOT_API std::string GetGroupName() const;
 
 private:
   std::string groupName_;

src/api/sorting/undefined_group_error.cpp

@@ -0,0 +1,34 @@
+/*  LOOT
+
+    A load order optimisation tool for Oblivion, Skyrim, Fallout 3 and
+    Fallout: New Vegas.
+
+    Copyright (C) 2018    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/>.
+    */
+#include "loot/exception/undefined_group_error.h"
+
+#include "api/sorting/plugin_graph.h"
+
+namespace loot {
+UndefinedGroupError::UndefinedGroupError(const std::string& groupName) :
+      std::runtime_error("The group \"" + groupName + "\" does not exist"),
+      groupName_(groupName) {}
+
+std::string UndefinedGroupError::GetGroupName() const { return groupName_; }
+}