[Fixed] Bootloader v0.10 broken on latest nightlies due to changes in the `Try` trait

[phil-opp]

CI run: https://github.com/rust-osdev/bootloader/runs/2629916522?check_suite_focus=true#step:9:275

[phil-opp]

I prepared a fix, but it is currently blocked on rust-osdev/uefi-rs#223.