[MRG] MAINT: Remove BaseLibLinear for LogisticRegression

BaseLibLinear has very little to do with liblinear in master.

This replaces it with a private `_fit_liblinear` function and simplifies the recursive code paths to an extent.
You should here include in the error message the value of 'solver' that was passed in, to help the user figuring out what he or she did wrong.