Added missing assert statement in test_temp_em() in test_goes.py
Removed unnecessary else: in time_intervals()
Explicitly indicated what type of exception is raised by each of
the functions in exceptions.py in their docstrings.
temp_em() in goes.py: changed getting date from time of first
measurement in GOESLightCurve to goeslc.meta["DATE-OBS"]
Made joining directory path and file names independent of OS by using
os.path.join in goes_get_chianti_temp(), goes_get_chianti_em() and
calc_rad_loss().
Changed print and error statements with inputs to string formatting.