Clarify failure in case of x/y building a too big matrix.

Example:
$ ffmpeg -hide_banner -f lavfi -i color=c=white:size=640x360,unsharp=lx=5:ly=23 -f null -t 1 -
[Parsed_unsharp_1 @ 0x5650e1c30240] luma matrix size (lx/2+ly/2)*2=26 greater than maximum value 25
color=c=white:size=640x360,unsharp=lx=5:ly=23: Invalid argument

Fix trac issue:
http://trac.ffmpeg.org/ticket/6033