Misleading error message on ENOENT

[katexochen]

When a given directory does not exist, the tool currently says it was not specified

strace overlay diff --lowerdir=/sysroot
...
readlink("/sysroot", 0x7ffdfc3f36c0, 1023) = -1 ENOENT (No such file or directory)
write(2, "Lower directory not specified.\n", 31Lower directory not specified.
) = 31

Would be nice to print that the directory does not exist instead.

Also, adding a --version flag and tagging releases would be a good idea. This tool is packaged for fedora, but it is impossible to say at what code state.

[UVV-gh]

Version is a good idea. Although I'd prefer to have a proper build system for that (I'm inclined to meson right now).

For the issue with the misleading message, could you please create a PR?

[katexochen]

I'll see if I can find the time for a PR. You could also add good-first-issue label.