Ensure Astro::Coord::ECI::TLE can raise errors

If the sgp4r method fails, the module attempts to produce an error
message including the time and the object ID.  If object ID is
not set an unnecessary warning is printed.  If the time is missing
then a misleading error is raised saying that the time is missing,
from a routine to which the time is passed!  The actual error
of interest is not reported in that case.  These problems can be
avoided by setting the time before calling sgp4r, and initializing
the object with a dummy object ID.
Add validation of the epoch parameters

If epoch is not present then check that epoch_year and epoch_day
are both defined.  Also check that the year is not before 1970
to avoid negative UNIX epoch numbers, and to catch cases in which
years are entered incorrectly with only the last two digits present.