853229 - blank sync plan date gives incorrect error

[jlsherrill]

Previously we relied on convert_date_time to throw an exception to catch. If an exception occurred we would leave date blank and let the model validation throw a validation error.

With this commit: 18d2fbe that was removed, so the exception convert_date_time threw simply went to the browser as a notification. This would also mean that invalid formats would show up with a warning that they couldn't be 'blank'.

So this commit has 2 parts, simply have convert_date_time return nil if either date or time is blank, so the model validation will get caught. If the conversion to a date object throws an exception however, we want to catch it and throw a more human readable exception. Ideally this would occur within the model and be a validaiton error, but that doesn't seem to be easy to do, since the model doesn't even have a time field, and the date field is something different completely.