backend: add tests for try-catch-finally and throwing exceptions

also add some tests for bugs with unreachable code handling and variables

backend.native/tests/codegen/try/finally4.kt

@@ -0,0 +1,25 @@
+fun main(args : Array<String>) {
+
+    try {
+        try {
+            println("Try")
+            throw Error("Error happens")
+            println("After throw")
+        } catch (e: Error) {
+            println("Catch")
+            throw Exception()
+            println("After throw")
+        } finally {
+            println("Finally")
+        }
+
+        println("After nested try")
+
+    } catch (e: Error) {
+        println("Caught Error")
+    } catch (e: Exception) {
+        println("Caught Exception")
+    }
+
+    println("Done")
+}