chore: avoid panic when parsing --dir argument

[Ocheretovich]

Avoid potential panic when parsing --dir argument by returning a proper error instead of unwrapping

[pchickey]

Did you find an input that can create this panic? From the docs of split

https://doc.rust-lang.org/std/primitive.str.html#method.split

Returns an iterator over substrings of this string slice, separated by characters matched by a pattern.
...
If there are no matches the full string slice is returned as the only item in the iterator.

by my reading, the iterator given by split will always contain at least one value, therefore the first call to next will always give Some.