added Fact check for Head/Tail

[filipw]

Added a fact check for Head/Tail array functions, which improves a bit the feedback.

let foo : Int[] = [];
let bar = Head(foo);

current error: ERROR: runtime error: index out of range: 0
new error: ERROR: runtime error: program failed: Array must have at least 1 element

let foo : Int[] = [];
let bar = Tail(foo);

current error: ERROR: runtime error: value cannot be used as an index: -1
new error: ERROR: runtime error: program failed: Array must have at least 1 element

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Regarding for the trailing whitespace changes - VS Code cleaned it up "on its own" due to "editor.formatOnType": true setting - but I kept them because they seem anyway to be consistent with the rest of the codebase.

[DmitryVasilevsky]

Approved and merged. Thank you!