nonexistent_local_time is thrown when
an attempt is made
to convert a non-existent local_time to a sys_time
without specifying choose::earliest or choose::latest.
Effects: Initializes the base class with a sequence of char
equivalent to that produced by os.str()
initialized as shown below:
ostringstream os;
os << tp <<" is in a gap between\n"<< local_seconds{i.first.end.time_since_epoch()}+ i.first.offset <<' '<< i.first.abbrev <<" and\n"<< local_seconds{i.second.begin.time_since_epoch()}+ i.second.offset <<' '<< i.second.abbrev
<<" which are both equivalent to\n"<< i.first.end <<" UTC";