fix axioms for extract, add extensionality checking for shared variables, convert exceptions to unknown status per #419

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/smt/smt_kernel.h

@@ -155,6 +155,12 @@ namespace smt {
         */
         std::string last_failure_as_string() const;
 
+
+        /**
+           \brief Set the reason for unknown.
+        */
+        void set_reason_unknown(char const* msg);
+
         /**
            \brief Return the set of formulas assigned by the kernel.
         */