`run fit() before predict()`: create new exception

[rlouf]

In the tests, we currently compare the exact text of the exception being raised. This is extremely precarious, as a change in the exception message needs to be propagated everywhere else in the library. It is easy to fix by creating a new exception.

[parsing-science]

I prefer to keep it as is. If you do change the message in an exception, I find it reasonable that you should think about how you have to change the unittests.