Runtime error from `Collage.gradient` given "illegal" `Color.linear` gradient

[jvoigtlaender]

With "elm-lang/core": "4.0.5" and "evancz/elm-graphics": "1.0.0", the following program results in a runtime error:

import Color
import Collage
import Element

grad =
    Color.linear
        ( 0, 60 )
        ( 0, -60 )
        [ ( 0, Color.white )
        , ( 2, Color.black )
        ]

main =
    Element.toHtml <|
        Collage.collage
            120
            120
            [ Collage.gradient grad (Collage.rect 120 120)
            ]

If one replaces the 2 in grad by something between 0 and 1, the program functions correctly.

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It's not unlikely that something similar would happen with Color.radial gradients.