improve error-handling in async function

fixes #3

async.lua

@@ -1,6 +1,4 @@
 
-local err_symbol = {}
-
 local function await(p)
     assert(coroutine.running(), "running inside a Promise.async() call")
     local result = nil
@@ -18,7 +16,7 @@ local function await(p)
     while true do
         if finished then
             if err then
-                return coroutine.yield({ err = err, err_symbol = err_symbol })
+                return nil, err
             else
                 return unpack(result)
             end
@@ -42,10 +40,6 @@ function Promise.async(fn)
             if not cont then
                 -- error in first async() level
                 p:reject(result)
-            end
-            if type(result) == "table" and result.err_symbol == err_symbol then
-                -- error in await() call
-                p:reject(result.err)
                 return
             end
             minetest.after(0, step)

async.spec.lua

@@ -32,25 +32,25 @@ end)
 
 mtt.register("Promise.async with handle_async", function()
     return Promise.async(function(await)
-        local v = await(Promise.resolved(42))
+        local v, err = await(Promise.resolved(42))
         assert(v == 42)
-        v = await(Promise.handle_async(function() return 100 end))
+        assert(not err)
+        v, err = await(Promise.handle_async(function() return 100 end))
         assert(v == 100)
+        assert(not err)
     end)
 end)
 
-mtt.register("Promise.async rejected", function(callback)
-    local p = Promise.async(function(await)
-        await(Promise.rejected("my-err"))
+mtt.register("Promise.async error propagation", function()
+    return Promise.async(function(await)
+        local v, err = await(Promise.rejected("my-err"))
+        assert(not v)
+        assert(err)
     end)
 
-    p:catch(function(e)
-        assert(type(e) == "string")
-        callback()
-    end)
 end)
 
-mtt.register("Promise.async error", function(callback)
+mtt.register("Promise.async direct error", function(callback)
     local p = Promise.async(function()
         error("stuff")
     end)