fix: add nil safety checks in ft.contains and ft.calculate

[yvng-jie]

• Add nil guard in ft.contains() to prevent 'attempt to index nil' error when treesitter tree is not yet parsed
• Add nil check for lang in ft.calculate() to fall back to filetype-based commentstring when treesitter parsing fails
• Fix error handler in utils.lua to use tostring(err) instead of err.msg, since Lua errors are strings not objects