Fix. Propagate error from reader.

```lua
function reader() return nil, "some error" end
local ok, err = arc:write('test.txt', fileDesc, reader)
assert(ok == nil)
assert(err == "some error")
-- !!! But zip archive contain test.txt file with zero size
```

lua/ZipWriter.lua

@@ -690,6 +690,7 @@ function ZipWriter:write(
   size   = 0
   csize  = 0
 
+  local reader_error -- error from reader (e.g. access error to file)
   if fileDesc.isfile then
     -- create stream for file data
     local stream = ZipWriter_as_stream(self)
@@ -730,6 +731,7 @@ function ZipWriter:write(
         size = size + #chunk
         chunk, ctx = reader(ctx)
       end
+      reader_error = ctx
     end
 
     csize = stream:close()
@@ -806,6 +808,9 @@ function ZipWriter:write(
     utfpath, cdextra, utfcomment
   )
   table.insert(self.private_.headers, cdh)
+
+  if reader_error then return nil, reader_error end
+
   return true
 end
 

test/test.lua

@@ -149,6 +149,48 @@ test_best    = Make('BEST')
 
 end
 
+local _ENV = TEST_CASE'ZipWriter reader interface' do
+
+function test_context()
+  local out = Stream:new()
+  local writer = ZipWriter.new()
+  writer:open_stream(out)
+
+  local function make_reader()
+    local ctx = {}
+    local i = 0
+    return function(o)
+      if i == 0 then
+        i = 1
+        assert_nil(o)
+        return "data", ctx
+      end
+      assert_equal(ctx, o)
+    end
+  end
+
+  assert_true(writer:write('test.txt', fileDesc, make_reader()))
+  assert_equal(1, writer:close())
+end
+
+function test_error()
+  local out = Stream:new()
+  local writer = ZipWriter.new()
+  writer:open_stream(out)
+
+  local ERR = {}
+
+  local function reader()
+    return nil, ERR
+  end
+
+  local ok, err = assert_nil(writer:write('test.txt', fileDesc, reader))
+  assert_equal(ERR, err)
+  assert_equal(1, writer:close())
+end
+
+end
+
 local _ENV = TEST_CASE'ZipWriter sink' do
 
 local function Make(lvl) return function()