Parse without seconds

[gregorp]

Not sure if this is a dupe of #30, but I'd expect as_hms to work without seconds, or at least give an intelligible warning or error.

hms::as_hms("08:00")
Error in `abort_lossy_cast()`:
! Lossy cast from <character> to <hms> at position(s) 1
Run `rlang::last_error()` to see where the error occurred.

Inspired by this Stack Overflow question where a user without seconds couldn't figure out what was going wrong.