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/catch8.kt

@@ -0,0 +1,10 @@
+fun main(args : Array<String>) {
+    try {
+        throw Error("Error happens")
+    } catch (e: Throwable) {
+        val message = e.message
+        if (message != null) {
+            println(message)
+        }
+    }
+}