Useless error message when loading a ragged image

[ttung]

Looks something like this:

self = <starfish.core.imagestack.parser.tileset._parser.TileSetData object at 0x132ffa0b8>, r = 1, ch = 0, z = 1

    def get_tile(self, r: int, ch: int, z: int) -> TileData:
        return SlicedImageTile(
>           self.tiles[TileKey(round=r, ch=ch, zplane=z)],
            r, ch, z,
        )
E       KeyError: (round: 1 ch: 0 z: 1)

starfish/core/imagestack/parser/tileset/_parser.py:123: KeyError

Ideally, we should make sure the tileset is "regular" before trying to load the data.