Improve default error messages for lollipop fields

[boris-42]

Current messages are not enough specific and don't actually help end user to figure out the problem.

For example if we use constant type:

>>> from lollipop import types
>>> x = types.Constant(2)
>>> x.validate(1)
'Value is incorrect'
# Instead of something more reasonable 
"Value 1 is incorrect, field is constant and can be only equal to 2"