IW-grammar-fix2: two more small tweaks to grammar-gen (#1192)
IW-grammar-fix2: two more small tweaks to grammar-gen (#1192)

IW-grammar-fix2: two small tweaks to the grammar-gen fix earlier this week:
1. A fix for statements that have 'show' in the middle, not the beginning
   (e.g. 'explain show tables').
2. Added two additional error messages to known_error_messages.
VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb,VoltDB/voltdb