Commit a62819e only addresses tables. Any other error not coerced to string, in
particular nil, can bust busted. The following test exposes the problem:

describe('busted', function ()
    it('must handle any error value', function () error() end)
end)

and results in:

[...]/utf_terminal.lua:22: attempt to concatenate field 'err' (a nil value)

This patch forcibly converts err to tostring(err) if it is not string or a
table onthe core side.

Do not abort on non-string errors

When installing via LuaRocks, the busted shell script and batch file fail to find busted_bootstrap (or rather, they find LuaRocks's own wrapper, and loading it fails because it is a shell script itself.) Simplifying the rockspec like this enables to use LuaRocks's own wrapper directly, and paths are properly setup.