Fix debug build by adding a missing conditional import

[ararslan]

src/lib/Err.hs conditionally uses unsafePerfomIO when DEX_DEBUG is defined, but it doesn't import System.IO.Unsafe, so unsafePerformIO is not defined. This can be fixed by importing that module when DEX_DEBUG is defined.

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[ararslan]

I keep getting a server error every time I try to sign the CLA. 🤔 Perhaps it'll work eventually.

EDIT: All good now

[ararslan]

@dougalm, I think this PR will fix CI, which is currently red on main. I believe CI is failing on main because setting DEX_CI enables DEX_DEBUG, which is broken without this change.