Make listing a non-created directory not throw an exception.

[BusterNeece]

For other adapters, if you list a directory that wasn't already created, the listContents call will simply return nothing; currently the Dropbox adapter throws a "not found" exception in this case, which (even though it makes sense in this context) makes its behavior at odds with the other adapters.

This PR just catches that exception and halts the generator early instead.

[freekmurze]

I'm not 100% sure that we should do this. If I'm not mistaken that Flysytem v2 now favours throwing exception instead of silently erroring.

What's your opinion about this @frankdejonge?

[frankdejonge]

@freekmurze for non-existent directories the listing should be empty, no exception needed. We model this after cloud filesystems where directories do not necessarily exist. It's basically a filter by prefix operation on all the entries of a filesystem.

TL;RD: looks good 👍

[freekmurze]

@SlvrEagle23 @frankdejonge Thanks!