Fix build with Qt >5.15.0

This fixes a regression from 3f430a23, which incorrecly tried to port purpose away from deprecated symbols.

While it's true that the QAbstractSocket::error signal was renamed to errorOccured and the latter added to QLocalSocket (which does not inherit from QAbstractSocket), QLocalSocket::error is something distinct and not deprecated (QLocalSocket also doesn't inherit from QAbstractSocket). QLocalSocket::socketError doesn't exist and breaks the build.