QAbstractSocket: Always set errorString right after socketError

Otherwise errorString() might not match error() in a
statusChanged handler.

Change-Id: If1250b4552412eabe88457dee031e4ef3f533838
Reviewed-by: Alex Trotsenko <alex1973tr@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>