Add new "404" tile structure to structure-mask.json

[Smasherrio]

#3730 New 404 Tile layout in Dungeons!

<Hey there devs!! I created a new tile structyre featuring the reknowned 404 Error Code in the dungeons layout code file. Am an avid player of CyberCode and would like this new dungeon layout to be introduced in the game. Thank you for your time. Hope you merge my request to the codebase>

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[Smasherrio]

Fixed structure to access entities spawning within 0 .

[lag145]

looks fine to me although for future refrence your original commit message is hard to read just "Adds dungeon layout featuring 404 motif" is fine you dont need to say how much of an avid player you are or how youd like it to be added to the game. or even where you added it as github shows all changes.

[Smasherrio]

Sorry for being over explanative as it was my first pr in this repo I chose to be more detailed. Will keep concise for future references

[lag145]

dw i just wanted to tell you for furure refrence as your pr was hard to read