Support CATCH on scalar expressions

Support

CATCH(expression, new-expression)

(where new-expression could be ERROR)

• Allows nested exceptions to be caught.

• Need to not perform cse between expressions inside a catch expression. (E.g., between two catch expressions/catch and recovery)

• May need changes to the engines to that exceptions in child queries are only reported when the result is accessed - providing an opportunity to catch it.

• • First job is to write the test cases... **