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/runtime/exceptions/catch1.kt

@@ -0,0 +1,16 @@
+fun main(args : Array<String>) {
+    try {
+        println("Before")
+        foo()
+        println("After")
+    } catch (e: Throwable) {
+        println("Caught Throwable")
+    }
+
+    println("Done")
+}
+
+fun foo() {
+    throw Error("Error happens")
+    println("After in foo()")
+}

backend.native/tests/runtime/exceptions/catch2.kt

@@ -0,0 +1,20 @@
+fun main(args : Array<String>) {
+    try {
+        println("Before")
+        foo()
+        println("After")
+    } catch (e: Exception) {
+        println("Caught Exception")
+    } catch (e: Error) {
+        println("Caught Error")
+    } catch (e: Throwable) {
+        println("Caught Throwable")
+    }
+
+    println("Done")
+}
+
+fun foo() {
+    throw Error("Error happens")
+    println("After in foo()")
+}

backend.native/tests/runtime/exceptions/catch7.kt

@@ -0,0 +1,14 @@
+fun main(args : Array<String>) {
+    try {
+        foo()
+    } catch (e: Throwable) {
+        val message = e.message
+        if (message != null) {
+            println(message)
+        }
+    }
+}
+
+fun foo() {
+    throw Error("Error happens")
+}