Detect invalid start and end times

If the user enters an invalid string such as "25:00" into the start time field
or the end/due time field, IncidenceDateTime does not detect it, because the
QDateTime constructor used by currentStartDateTime() and currentEndDateTime()
treats it as "00:00".

This patch
 * detects and reports the error,
 * uses more specific error messages,
 * focuses on the erroneous field, and
 * logs at "debug" level, because the problem isn't very big.

CCBUG: 409912