Tzdata dependency is required and its absence should be handled

[mindseyeblind]

The function do_handle_datetime/3 uses DateTime.from_naive/3 to convert datetimes from naive to utc. Elixir by default does not ship with a timezone db, so attempting to use this function without a timezone database installed gives an error of {:error, :utc_only_time_zone_database} which then causes the function to crash.

Solutions to prevent crashing:

• Add a clause to do_handle_datetime/3 handle the {:error, :utc_only_time_zone_database} output of DateTime.from_naive/3.
• Add a clause to handle the likewise unhandled output {:error, :incompatible_calendars}

Solutions for timezone dependency:

• Add a timezone database as strict dependency e.g. tzdata OR
• Instruct the user to install a timezone dependency.

[LostKobrakai]

I guess the first one should stick to crashing as the library is not intended to be used without a timezone database configured (which should however be documented) and for the second one I guess it makes sense to use an optional callback. For as long as the input_datetime is cast by ecto I'm not sure it can even be in a calendar different to Calendar.ISO.

[LostKobrakai]

Implemented the changes like described.