DatArchive.fork error handling

[sammacbeth]

When DatArchive.fork is called with incorrect arguments the dialog still appears, but in a broken state. We should just reject immediately.

Reproduction:
The self-mutating site example calls DatArchive.fork with a DatArchive as the first argument (this is perhaps an older version of the API). This may be a case we should handle for backwards compatibility, but it demonstrates this issue at present.