ENH FIX Raise ValueError when the target does not match with classes arg

ENH Use np.searchsorted to find the index instead of cls2idx
TST Added NRT to test if ValueError is raised during target mismatch.

This test will be removed when a more general test is added.
MAINT: fix silhouette typo
Merge pull request #3974 from lesteve/fix-silhouette-typo

[MRG] Fix silhouette typo
MAINT remove probability kwarg from SVR and NuSVR

These don't have predict_proba, so setting it to true makes no sense.
Merge branch 'pr/3944'
TST remove SVR(probability=True) from AdaBoost tests

Doesn't seem to have been testing anything, because CustomSVR doesn't
have predict_proba.