Add `try`/`except` around calls to `pypeit.core.fittting.iterfit()`

[tbowers7]

In the fitting function iterfit(), a ValueError is raised if there are no valid data points for the fitting:

PypeIt/pypeit/core/fitting.py

Lines 952 to 955 in 81057b0

	else:
	if not maskwork.any():
	raise ValueError('No valid data points.')
	# return (None,None)

This should be caught by the calling functions and should yield a context-specific error message that will help the user diagnose why there were no valid points in that particular fit. As the code stands, the above lines result in a semi-useless traceback. For consideration, this function also raises a ValueError if various inputs do not match length as they should -- which is a coding error rather than a problem with the input data. I suggest the above lines raise a different type of error that can be caught by the calling functions (perhaps a core-specific custom Exception).

---

As of the creation of this issue, pypeit.core.fittting.iterfit() is called 5 times from 2 different modules:

PypeIt/pypeit/core/extract.py

Lines 900 to 905 in 81057b0

	b_answer, bmask = fitting.iterfit(wave[indsp], flux_sm[indsp], invvar = fluxivar_sm[indsp],
	kwargs_bspline={'everyn': 1.5}, kwargs_reject={'groupbadpix':True,'maxrej':1})
	b_answer, bmask2 = fitting.iterfit(wave[indsp], flux_sm[indsp], invvar = fluxivar_sm[indsp]*bmask,
	kwargs_bspline={'everyn': 1.5}, kwargs_reject={'groupbadpix':True,'maxrej':1})
	c_answer, cmask = fitting.iterfit(wave[indsp], flux_sm[indsp], invvar = fluxivar_sm[indsp]*bmask2,
	kwargs_bspline={'everyn': 30}, kwargs_reject={'groupbadpix':True,'maxrej':1})

PypeIt/pypeit/core/extract.py

Lines 1055 to 1056 in 81057b0

	bset, bmask = fitting.iterfit(sigma_x.flat[si],norm_obj.flat[si], invvar = norm_ivar.flat[si],
	nord = 4, bkpt = bkpt, maxiter = 15, upper = 1, lower = 1)

PypeIt/pypeit/core/flux_calib.py

Lines 1023 to 1029 in 81057b0

	# init_breakpoints = fullbkpt
	msgs.info("Bspline fit on zeropoint. ")
	bset1, bmask = fitting.iterfit(wave, zeropoint_clean, invvar=zeropoint_ivar, inmask=zeropoint_fitmask, upper=upper, lower=lower,
	fullbkpt=init_breakpoints, maxiter=maxiter, kwargs_bspline=kwargs_bspline,
	kwargs_reject=kwargs_reject)
	zeropoint_bspl, zeropoint_fit_gpm = bset1.value(wave)
	zeropoint_bspl_bkpt, _ = bset1.value(init_breakpoints)