verifies the variable extid is actually an extension ID
makes sure typing an invalid input doesnt remove your perms for your user's entire extension folder (since there was no check on the extid variable, it would run chmod 000 on "/home/chronos/user/Extensions/$extid" even if $extid is unset, meaning the chmod command was run on the "/home/chronos/user/Extensions/" directory as a whole)
fixes two things:
extid
is actually an extension IDextid
variable, it would runchmod 000
on "/home/chronos/user/Extensions/$extid" even if $extid is unset, meaning the chmod command was run on the "/home/chronos/user/Extensions/" directory as a whole)