make AbstractNetworkJob::errorString virtual: it is already overriden

in practice AbstractNetworkJob::errorString is already overriden but the
overrided code is probably never called while the intention looked like
the opposite

fix that by making the method virtual in base class

Signed-off-by: Matthieu Gallien <matthieu.gallien@nextcloud.com>

src/libsync/abstractnetworkjob.h

@@ -77,7 +77,7 @@ public:
     bool timedOut() const { return _timedout; }
 
     /** Returns an error message, if any. */
-    QString errorString() const;
+    virtual QString errorString() const;
 
     /** Like errorString, but also checking the reply body for information.
      *

src/libsync/propagateupload.h

@@ -131,7 +131,7 @@ public:
         return _device;
     }
 
-    QString errorString()
+    QString errorString() const override
     {
         return _errorString.isEmpty() ? AbstractNetworkJob::errorString() : _errorString;
     }