when:so does not fail on all malformed date-times

[jackfoxy]

Describe the bug
When parsing date-time with when:so and the fractional seconds are malformed it truncates the result to the hour/minute/second component.
When hour/minute/second is malformed it truncates to date.

To Reproduce
(wonk (when:so [[1 1] "2020.12.25..7.15.1..9ef"]))
[[%.y 2.020] 12 25 [7 15 1 ~]]

(wonk (when:so [[1 1] "2020.12.25..7.15."]))
[[%.y 2.020] 12 25 [0 0 0 ~]]

Expected behaviour
hoon expression failed

The fix
Change 2 LOC in hoon.hoon when:so

++  when
  ~+
  ;~  plug
    %+  cook
      |=([a=@ b=?] [b a])
    ;~(plug dim:ag ;~(pose (cold | hep) (easy &)))
    ;~(pfix dot mot:ag)   ::  month
    ;~(pfix dot dip:ag)   ::  day
    ;~  pose
      ;~  pfix
        ;~(plug dot dot)
        ;~  plug
          dum:ag
          ;~(pfix dot dum:ag)
          ;~(pfix dot dum:ag)
::          ;~(pose ;~(pfix ;~(plug dot dot) (most dot qix:ab)) (easy ~))
          ;~(pose ;~(pfix ;~(plug dot dot) (most dot qix:ab)) ;~(less dot (easy ~)))
        ==
      ==
::      (easy [0 0 0 ~])
      ;~(less dot (easy [0 0 0 ~]))
    ==
  ==

[jackfoxy]

Unit tests: I don't see an existing file appropriate for unit testing this.
I suggest adding time.hoon to tests/sys, or make a new directory path in tests. This file would be appropriate for testing time and time-range parsing and manipulations.

Happy to submit a PR for this issue.