Replace error types with Unrecognized() variant

[MarijnS95]

As #17 (comment) noticed, only DrmModifier has an Unrecognized() variant, which allows more transparently passing around custom or yet-unknown-to-drm-fourcc-rs modifiers. The same seems to apply to DrmFourcc and DrmVendor, where new constants aren't added that infrequently.

By having an Unrecognized() variant users can freely store a DrmFourcc/DrmVendor and pass it around, without treating new values as a failure case.

Important

While just fixing up the tests after initially making the change, I did realize that a value of 0 is invalid, unlike DrmModifier which has a LINEAR variant. Perhaps this is worth keeping the error parser for, and this change should show that rewriting the original error tests for this became awkward.

[MarijnS95]

One more advantage is that because fourcc_display_form() isn't public, callers have to write quite a few lines of code to display the four-character code nicely based on DrmFourcc::try_from(), or Debug-print that Result directly. That should no longer be necessary with this change.