vm: Save current active exception on opening new try block.

Required to reraise correct exceptions in except block, regardless if more
try blocks with active exceptions happen in the same except block.

P.S. This "automagic reraise" appears to be quite wasteful feature of Python
- we need to save pending exception just in case it *might* be reraised.
Instead, programmer could explcitly capture exception to a variable using
"except ... as var", and reraise that. So, consider disabling argless raise
support as an optimization.

py/bc.h

@@ -9,6 +9,8 @@ typedef struct _mp_exc_stack {
     const byte *handler;
     // bit 0 is saved currently_in_except_block value
     mp_obj_t *val_sp;
+    // Saved exception, valid if currently_in_except_block bit is 1
+    mp_obj_t prev_exc;
     // We might only have 2 interesting cases here: SETUP_EXCEPT & SETUP_FINALLY,
     // consider storing it in bit 1 of val_sp. TODO: SETUP_WITH?
     byte opcode;

py/vm.c

@@ -51,10 +51,12 @@ typedef enum {
     exc_sp->opcode = op; \
     exc_sp->handler = ip + unum; \
     exc_sp->val_sp = MP_TAGPTR_MAKE(sp, currently_in_except_block); \
+    exc_sp->prev_exc = nlr.ret_val; \
     currently_in_except_block = 0; /* in a try block now */
 
 #define POP_EXC_BLOCK() \
     currently_in_except_block = MP_TAGPTR_TAG(exc_sp->val_sp); /* restore previous state */ \
+    if (currently_in_except_block) { nlr.ret_val = exc_sp->prev_exc; } \
     exc_sp--; /* pop back to previous exception handler */
 
 mp_vm_return_kind_t mp_execute_byte_code(const byte *code, const mp_obj_t *args, uint n_args, const mp_obj_t *args2, uint n_args2, mp_obj_t *ret) {

tests/basics/try-reraise2.py

@@ -0,0 +1,23 @@
+# Reraise not the latest occured exception
+def f():
+    try:
+        raise ValueError("val", 3)
+    except:
+        try:
+            raise TypeError
+        except:
+            try:
+                try:
+                    raise AttributeError
+                except:
+                    pass
+                raise
+            except TypeError:
+                pass
+        # This should raise original ValueError, not the most recently occurred AttributeError
+        raise
+
+try:
+    f()
+except ValueError as e:
+    print(repr(e))