raise ValueError if dichotomy fails

[jmmshn]

Address this issue from solid_dmft
solid_dmft #100
dochotomy can fail sometime and will return None, but the result will be sent to chemical_potential which causes downstream errors.
Here, we raise an exception and let the user know how it might have failed.

[the-hampel]

Thank you @jmmshn . This looks good to me. I just checked and the other two available methods newton and brenth from scipy will throw an exception if they failed by themselves. So we do not need to catch those.