`fit_salt` can return unconverged lightcurve fits

[AntoineGillesLordet]

I noticed that in skysurvey/lcfit/sncutils.py, the obtained result from sncosmo.fit_lc is converted to a dataframe without any check.

Since sncosmo uses iminuit to fit the lightcurves, it will always return the ending point of the minimization whatever its status is, so assuming that the fit converged if it didn't raise any errors is wrong.

Instead the Result object returned also contains a success attribute to tell wether the fit actually converged and additionnal informations about it in message.