Improve error messages

[prostomarkeloff]

now it's not really informative:

betterconf/betterconf/_config.py

Line 127 in 068faac

"Something bad happened.\nBetterconf can't deal with this kind of value.\nProbably you've tried to use something like 'dict' in a constant manner. For this special case use 'constant_field`"

TODO: improve!!