Recognize YCbCr/Chomra subsampling tags

In particular we avoid mistaking the absence of this tag for an absence
of subsampling. The specification instead requires it to mean half the
samples in both directions—a mode that we do not support yet. Instead of
returning erroneous samples or an error in the data stream (which would
appear too short) we return an error when asked to layout the read
buffer for these images.

src/error.rs

@@ -90,6 +90,9 @@ quick_error! {
         InvalidTypeForTag {
             display("tag has invalid type")
         }
+        InvalidCountForTag(tag: Tag, len: usize) {
+            display("tag `{tag:?}` with incorrect number of elements ({len}) encountered")
+        }
         StripTileTagConflict {
             display("file should contain either (StripByteCounts and StripOffsets) or (TileByteCounts and TileOffsets), other combination was found")
         }
@@ -156,6 +159,9 @@ quick_error! {
         UnsupportedInterpretation(interpretation: PhotometricInterpretation) {
             display("unsupported photometric interpretation \"{interpretation:?}\"")
         }
+        ChromaSubsampling {
+            display("chroma subsampling of YCbCr color is unsupported")
+        }
         MisalignedTileBoundaries {
             display("tile rows are not aligned to byte boundaries")
         }