Throw a runtime error when trying to slice an unsupported type

[mkuznets]

Add a missing switch case corresponding to a slice of an unsupported type. Without it Tengo continues execution past the unsupported slice with an invalid stack state, causing unexpected behaviour or obscure errors.

Example:

test := func() {
    a := 42
    return undefined[:5]
}

fmt := import("fmt")
fmt.println(test())  // -> prints 42

This PR changes it to throw a runtime error instead, which is consistent with similar cases such as using a function call operator on an unsupported type.

[geseq]

Thanks for the PR. LGTM.