menubar: assert() that io.open() succeeds

Improves-the-error-for: https://github.com/awesomeWM/awesome/issues/1663
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/menubar/init.lua

@@ -146,7 +146,7 @@ end
 local function write_count_table(count_table)
     count_table = count_table or instance.count_table
     local count_file_name = gfs.get_dir("cache") .. "/menu_count_file"
-    local count_file = io.open (count_file_name, "w")
+    local count_file = assert(io.open(count_file_name, "w"))
     for name, count in pairs(count_table) do
         local str = string.format("%s;%d\n", name, count)
         count_file:write(str)