mcookie: generate cookie even if specified file doesn't exist

[cakebaker]

The original mcookie shows an error message if the specified file doesn't exist, but still generates a cookie and returns an exit code of 0:

$ mcookie -f non-existing
mcookie: cannot open non-existing: No such file or directory
2a40970ac734f28034a8bcabf00b7026
$ echo $?
0

Our implementation, on the other hand, shows an error message, but doesn't generate a cookie and returns an exit code of 1:

$ cargo run -q mcookie -f non-existing
mcookie: No such file or directory
$ echo $?
1