Fix #1023: Deprecate implicit conversions to/from js.Array

The deprecation message should start with something like "Implicit conversion of scala.Array to js.Array is deprecated. Use _.toJSArray ..."
That's because it will be reported in locations where `fromArray` is not even written. So if the deprecation message is not a bit more explicit, the error message is very obscure.

Same for the other one, obviously.