minor: write reader error to stderr

frontend/apps/reader/readerui.lua

@@ -346,8 +346,8 @@ function ReaderUI:showReader(file)
         end)
         local ok, err = coroutine.resume(co)
         if err ~= nil or ok == false then
-            print '[!] doShowReader coroutine crashed:'
-            print(debug.traceback(co, err, 1))
+            io.stderr:write('[!] doShowReader coroutine crashed:\n')
+            io.stderr:write(debug.traceback(co, err, 1))
             UIManager:quit()
         end
     end)

frontend/ui/widget/horizontalgroup.lua

@@ -40,7 +40,8 @@ function HorizontalGroup:paintTo(bb, x, y)
         elseif self.align == "bottom" then
             widget:paintTo(bb, x + self._offsets[i].x, y + size.h - self._offsets[i].y)
         else
-            print("[!] invalid alignment for HorizontalGroup", self.align)
+            io.stderr:write("[!] invalid alignment for HorizontalGroup: ",
+                            self.align)
         end
     end
 end