Implement model error dialog box for open config file failure

Also corrects unused variable warning on Linux compilation

Author: jamescowens

src/qt/bitcoingui.cpp

@@ -748,18 +748,24 @@ void BitcoinGUI::openConfigClicked()
     boost::filesystem::path pathConfig = GetConfigFile();
     /* Open gridcoinresearch.conf with the associated application */
     bool res = QDesktopServices::openUrl(QUrl::fromLocalFile(QString::fromStdString(pathConfig.string())));
-    #ifdef Q_OS_WIN
+
+#ifdef Q_OS_WIN
     // Workaround for windows specific behaviour
     if(!res) {
         res = QProcess::startDetached("C:\\Windows\\system32\\notepad.exe", QStringList{QString::fromStdString(pathConfig.string())});
     }
-    #endif
-    #ifdef Q_OS_MAC
+#endif
+#ifdef Q_OS_MAC
     // Workaround for macOS-specific behaviour; see https://github.com/bitcoin/bitcoin/issues/15409
     if (!res) {
         res = QProcess::startDetached("/usr/bin/open", QStringList{"-t", QString::fromStdString(pathConfig.string())});
     }
-    #endif
+#endif
+
+    if (!res) {
+        error("File Association Error", "Unable to open the config file. Please check your operating system"
+                                        " file associations.", true);
+    }
 }
 
 void BitcoinGUI::researcherClicked()