[FEATURE] Throw an error when the morphed target is not a valid SVGPolygonElement or SVGPathElement

[juliangarnier]

At the moment, morphing an invalid target type results in a vague error message like mentioned in #1060

Console error reads: TypeError: $path1.getTotalLength is not a function. (In '$path1.getTotalLength()', '$path1.getTotalLength' is undefined)

We should throw a much more precise error, explaining which type of target can be morphed.