fcl-db: reverts rev.29118, because there are 3rd party libraries (like ZEOS), which define own EDatabaseError descendants, where they define members FErrorCode and/or ErrorCode.
This patch also fixes bugs #27078 and #26684, what was intention of rev.29118

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@29163 3ad0048d-3df7-0310-abae-a5850022a9f2