Decide on names for icu_datetime and icu_calendar errors over FFI

[sffc]

icu_datetime and icu_calendar have many new, small error enums. We should discuss their naming conventions.

One example which isn't super great is that icu_calendar::ParseError is currently exported as CalendarParseError, even though it is the error resulting from parsing an IXDTF string to a Date, Time, or DateTime. (#5260)

[sffc]

I want to save this discussion until after neo datetime is fully landed so that we can see exactly which error enums we're having to name in FFI.

[sffc]

Part of the 2.0 FFI audit

[sffc]

@robertbastian says he's fine with names so long as they are targeted/descriptive as opposed to reliant on the crate name.

[Manishearth]

This is blocked on #5940

We don't seem to need a discussion yet, Shane plans to make a PR when ready and we can discuss if there is controversy.