datetime: raise error for ambiguous, nyi timezones

Use different return values for indication that recognized
timezone suffix is ambiguous or not yet handled (before tzdata
integration to come in next patchset).

i.e. we could always parse '2020-02-02T01:03 MSK' as MSK is
static +03:00, but it's ambiguous if there is something like
'2020-02-02T01:03 AT' as there are multiple ATs (e.g.
'Alaska Time' and 'Atlantic Time').

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