No error maps generated when non-existent save directory is given

[xge]

I stumbled across this very minor nuisance when using flip: Specifying an output directory for the error maps using the -d parameter, flip fails silently if the directory does not exist. This results in no error maps being saved.

Example: Running flip-cuda.exe -d output -r reference.png -t test.png will not create the output directory, but instead flip will print the metrics and then exit.

Expected behavior: Either create the directory or print a warning that the error maps will not be saved.

[inversepixel]

On it!

[inversepixel]

Done!

[inversepixel]

Note that if you use \ in your filename you need to use two of them, i.e., --directory mydir\\subdir