Inconsistent API between first, last, single

[Celebes]

single has an OK API:

• emits value if there's only one or one that meets the requirements of predicate
• throws EmptyError when no values are emitted
• throws NotFoundError when values are emitted, but none meet requirements
• throws SequenceError when too much values are emitted

But first() and last() throw EmptyError always, instead of NotFoundError when predicate is added