pgsql: Improve error message when an FDW doesn't support WHERE CURRENT

Improve error message when an FDW doesn't support WHERE CURRENT OF.

If an FDW fails to take special measures with a CurrentOfExpr, we will
end up trying to execute it as an ordinary qual, which was being treated
as a purely internal failure condition.  Provide a more user-oriented
error message for such cases.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6e481ebff6368cb0ab5351a5ef3463747c35af22

Modified Files
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src/backend/executor/execQual.c |   12 ++++++++----
1 files changed, 8 insertions(+), 4 deletions(-)