defined a command to return `false` when an element does not exist

[KazuCocoa]

flutter: isEnabled or something.

#693 (comment) does as part of this flutter driver's command.

[agataciesielska]

This!
Really struggling to understand what is the point of returning random id when command find.by[anything] doesn't find element.

[SaurabhBlox]

This is a big problem if assertion is not present in flutter