Throw graceful error for malformed time or date strings

[PhantomWatson]

Throw graceful errors for

• Date strings that aren't in YYYY-MM-DD format
• Date strings that are out of bounds (too far in past or future, or describing days that don't exist)
• Time strings that fail being translated to FrozenTime objects

[PhantomWatson]

Note: Errors were thrown recently because start_time was being passed as a datetime string. In the back end, time objects are constructed from the provided date string concatenated with the provided time string, so this caused redundant dates to be crammed into that constructor.

2019-04-08 14:50:45 Error: [Exception] DateTimeImmutable::__construct(): Failed to parse time string (4/15/19 2019-04-15T23:00:00.000Z) at position 8 (2): Double date specification