Tweak error message on unsupported statement.
Tweak error message on unsupported statement.

Use 'column' instead of 'key'. Mention equality condition.

deerwalk/voltdb,zuowang/voltdb,kumarrus/voltdb,kobronson/cs-voltdb,simonzhangsm/voltdb,paulmartel/voltdb,simonzhangsm/voltdb,simonzhangsm/voltdb,ingted/voltdb,creative-quant/voltdb,simonzhangsm/voltdb,VoltDB/voltdb,migue/voltdb,creative-quant/voltdb,creative-quant/voltdb,simonzhangsm/voltdb,ingted/voltdb,simonzhangsm/voltdb,kobronson/cs-voltdb,kobronson/cs-voltdb,zuowang/voltdb,creative-quant/voltdb,ingted/voltdb,kumarrus/voltdb,paulmartel/voltdb,migue/voltdb,zuowang/voltdb,creative-quant/voltdb,ingted/voltdb,wolffcm/voltdb,creative-quant/voltdb,VoltDB/voltdb,ingted/voltdb,paulmartel/voltdb,paulmartel/voltdb,kumarrus/voltdb,deerwalk/voltdb,wolffcm/voltdb,wolffcm/voltdb,simonzhangsm/voltdb,simonzhangsm/voltdb,zuowang/voltdb,flybird119/voltdb,flybird119/voltdb,deerwalk/voltdb,deerwalk/voltdb,ingted/voltdb,kobronson/cs-voltdb,creative-quant/voltdb,ingted/voltdb,deerwalk/voltdb,kumarrus/voltdb,wolffcm/voltdb,zuowang/voltdb,deerwalk/voltdb,wolffcm/voltdb,kobronson/cs-voltdb,kumarrus/voltdb,wolffcm/voltdb,deerwalk/voltdb,migue/voltdb,wolffcm/voltdb,migue/voltdb,zuowang/voltdb,migue/voltdb,kobronson/cs-voltdb,flybird119/voltdb,VoltDB/voltdb,deerwalk/voltdb,flybird119/voltdb,zuowang/voltdb,paulmartel/voltdb,creative-quant/voltdb,VoltDB/voltdb,kumarrus/voltdb,kobronson/cs-voltdb,paulmartel/voltdb,VoltDB/voltdb,migue/voltdb,zuowang/voltdb,VoltDB/voltdb,flybird119/voltdb,flybird119/voltdb,VoltDB/voltdb,wolffcm/voltdb,kumarrus/voltdb,migue/voltdb,ingted/voltdb,paulmartel/voltdb,flybird119/voltdb,migue/voltdb,kumarrus/voltdb,paulmartel/voltdb,flybird119/voltdb,kobronson/cs-voltdb