Summoning a mannequin with an invalid texture will display the missing texture rather than falling back to a default skin

When summoning a mannequin with an invalid profile, one of the default player skins will be used as a fallback. This is correct behaviour. However, summoning a mannequin with an invalid texture entry in a mannequin profile will result in the missing texture displaying, rather than using a default skin as a fallback.

Commands to reproduce:

/summon minecraft:mannequin ~ ~ ~ {profile:thisusernameisinvalidanddoesnotbelongtoanyaccount}

/summon minecraft:mannequin ~ ~ ~ {profile:{texture:"thisisatexturethatdoesnotexist"}}

Expected results: both commands result in usage of one of the default skins.

Actual results: the first correctly does that, but the second shows the missing texture (as though there is an issue with the current resource pack).

To fix: if a texture cannot be loaded, default to one of the nine player skins like is done with online profiles that cannot be loaded.