Return error when files are missing

[ineiti]

Currently the API will probably panic if the files are not in the directory. It should return correct errors for:

• no directory passed (in the best case, it should just do the normal tor-thingie)
• directory empty or not existing
• file(s) missing

[wouterl]

Does this also properly catch if the directory is empty?

[wouterl]

@laurent-girod can you also add a small test to arti-rest itself?

[cgrigis]

Tests on the android side added with c4dt/lightarti-rest-android#5

[ineiti]

iOS is failing with opaque error - needs to be fixed by @ineiti