Fix bug causing error message for incorrect POS not to be shown
Allow unknown items in D-Scan

There was an odd bug where unreconigsed types were being shown on a d-scan paste
which caused the POS.fit_from_iterable method to throw an exception. Unknown
items are now ignored.
Improve error handling with auto POS function

If there were multiple POSs or no POSs the same error message was displayed
previously. Exceptions thrown in POS.fit_from_iterable were also being
incorrectly caught which made debugging harder (wrong error messages).
Merge pull request #77 from joshuablake/auto_pos

Improve Error Handling in Auto POS adder