Reduce variants from DateTimeWriteError

[sffc]

As of #6233, we use the same error enum in 3 places:

1. FormattedDateTimePattern
2. FormattedDateTimeUnchecked
3. FFI ZonedDateTimeFormatter

However, the error cases are not the same across all 3.

[sffc]

@robertbastian wants to be a reviewer on this.