Refactored VCardFormatError into subclasses

- `VCardFormatError` renamed to `VCardError`
- Error Subclasses for Names, Values, and ItemCount

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Dunno what you’ll think of this one, @l0b0, but I thought some error types which describe more specific types of VCard errors would be helpful for the end-user.  

The classes  `VCardNameError` and `VCardValueError` are a nice parallel to the standard library exceptions `NameError` and `ValueError` (although they only inherit from `VCardError`).  

The class `VCardItemCountError` is vaguely like the standard library `IndexError`, in that both are raised when an expected number of something is out of bounds.

Finally, there’s the `VCardLineError` class, whose name makes it pretty obvious what went wrong. :)