4xm: return meaningful error codes
4xm: simplify code with FFSWAP
4xm: eliminate a pointless indirection
4xm: operate with pointers to AVFrames instead of whole structs.

This is more correct and avoids breaking extended_data.
8bps: return meaningful error codes.
aasc: return meaningful error codes.
anm: return meaningful error codes
ansi: return a meaningful error code
asvdec: return meaningful error codes.
aura: return meaningful error codes.
avs: return meaningful error codes.
bethsoftvideo: return meaningful error codes.
bfi: return meaningful error codes.
bink: return meaningful error codes.
bink: operate with pointers to AVFrames instead of whole structs.

This is more correct and avoids breaking extended_data.
bmp: return meaningful error codes.
bmv: return meaningful error codes.
c93: return meaningful error codes.
yop: use a meaningful error code.
zmbv: return more meaningful error codes.