Feature request: raise Exception when an item in distributions is not a valid distribution

[amirpaster]

When an item in the distributions list is not a valid distribution, the fit method yields warnings, e.g.:
WARNING:root:SKIPPED normal distribution (taking more than 30 seconds)
WARNING:fitter.fitter:normal was not fitted. no parameters available
These are IMHO ambiguous and should be replaced by raising an exception

The above behaviour can be generated by the following commands
f = Fitter(data, distributions=['beta', 'gamma', 'normal'])
f.fit()
which yield the following result https://gist.github.com/amirpaster/c25bac5e4cacae04fe538742f8b6e1cf