Update model.rb - Throw 'UnknownDnsResourceError'

Throw `UnknownDnsResourceError` instead of `InvalidDnsResourceError`

Prevents `[20:30:55][!] Internal error while adding DNS rule (uninitialized constant BeEF::Core::Models::Dns::Rule::InvalidDnsResourceError)` for invalid user supplied DNS response types.

'BeEF::Core::Models::Dns::Rule::InvalidDnsResourceError' does not exist, and it's unlikely we'll need to differentiate between invalid and unknown resource types.
Merge pull request #2 from bcoles/patch-2

Update model.rb - Throw 'UnknownDnsResourceError'