Throw specific exception when creating empty single file archive

[plamentotev]

Currently archivers that accept single file (like BZip2Archiver) will throw NoSuchElementException.
Modify them to throw EmptyArchiveException so they are consistent with DirectoryArchiver, TarArchiver and ZipArchiver.

[michael-o]

Looks good to me, but the exception message does not correspond to the existing ones.

[plamentotev]

Looks good to me, but the exception message does not correspond to the existing ones.

Do you mean "You must set at least one file."? I initially copy-pasted the message from the other archivers, but then I thought that it's a bit confusing. "... at least one file" suggests that you can set two or more files and that is not the case. That's why I've changed it to something that says that you should set a single file. Unfortunately my English is long way from perfect, so if you have suggestion I'll gladly change the exception message.

[michael-o]

Well, then this is pretty much the same as String.isEmpty() => throw new IllegalArgumentException("archive cannot be empty"). Done.

[plamentotev]

@michael-o sounds good, thanks for the suggestion. I've just updated the exception message.