Better error messaging when source is undefined

[joshbruce]

Perhaps add some error reporting for marked() if src ends up undefined 🤔? That way it's pretty clear the issue is not the marked library...

See #969

[Feder1co5oave]

My #682 tried to fix this by silently failing (return null), though I agree with you that a more explicit error message can be more helpful as rightfully pointed out here: #682 (comment).

Maybe something along the lines of
marked(): input parameter is null/undefined or
marked(): input parameter of type <type>, string expected

?

[Feder1co5oave]

Can close this