Show error message if ReadConfigFile fails

A runaway exception was raised if ReadConfigFile fails (usually
due to a parse error in bitcoin.conf). Show an error message instead.

Fixes #4013.

src/qt/bitcoin.cpp

@@ -507,7 +507,13 @@ int main(int argc, char *argv[])
                               QObject::tr("Error: Specified data directory \"%1\" does not exist.").arg(QString::fromStdString(mapArgs["-datadir"])));
         return 1;
     }
-    ReadConfigFile(mapArgs, mapMultiArgs);
+    try {
+        ReadConfigFile(mapArgs, mapMultiArgs);
+    } catch(std::exception &e) {
+        QMessageBox::critical(0, QObject::tr("Bitcoin"),
+                              QObject::tr("Error: Cannot parse configuration file: %1. Only use key=value syntax.").arg(e.what()));
+        return false;
+    }
 
     /// 7. Determine network (and switch to network specific options)
     // - Do not call Params() before this step