Tidy up idiokit.dns._dns exceptions

[jviide]

This pull request does some minor cleanups on how idiokit.dns._dns raises exceptions:

• The unpack classmethod for A, AAAA and TXT types should raise NotEnoughData exceptions like other similar methods when there is not enough data.
• Instead of checking the string lengths during TXT#pack the lengths should be checked when TXT is instantiated do better pinpoint the actual source of erroneous data.

Relevant tests are included.