file:write() fails silently for read-only files

[HyperHacker <hyperhacker@...>]

Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> f=io.open('foo.foo', 'r')
> f:write('ffd')
> f:close()
>

Call me crazy, but I feel like f:write() should throw an error when
trying to write to a read-only file, instead of just doing nothing.

-- 
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