sbchooser: fix load_secdb_from_file returning fd instead of rc

[adsuna]

When read_file() fails, returning fd instead of rc causes callers to miss the error since fd is a non-negative integer after close(). Fix by returning rc.
This affects when an invalid path is passed as db where the output shows not trusted rather than a read error.

# bugged output
LD_LIBRARY_PATH=src src/sbchooser --explain -d /tmp -i tests/shim-16.1-4.el10.x64.msft2023.efi -i tests/shim-16.1-4.el10.x64.msft2011.efi; echo "exit code: $?"
tests/shim-16.1-4.el10.x64.msft2023.efi is not trusted because no certs or hashes trust it
tests/shim-16.1-4.el10.x64.msft2011.efi is not trusted because no certs or hashes trust it
exit code: 0

# expected output
LD_LIBRARY_PATH=src src/sbchooser --explain -d /tmp -i tests/shim-16.1-4.el10.x64.msft2023.efi -i tests/shim-16.1-4.el10.x64.msft2011.efi; echo "exit code: $?"
sbchooser: Could not load db from "/tmp": Is a directory
exit code: 2