Raise "unknown OSC type" when an unknown OSC tag is encountered.

Current behaviour results in a mystical error message:

`undefined method `call' for nil:NilClass (NoMethodError)`

This patch makes that error slightly more readable and understandable.
converting project to minispec
Merge branch 'handle-unknown-type' of https://github.com/samaaron/osc-ruby into samaaron-handle-unknown-type
creating a custom exception class for unknown type errors. adding tests.