str() on a FixedLengthError instance crashes with AttributeError

[dargueta]

• FWFFR version: 0.4.0
• Python version: 3.7
• Operating System: Ubuntu, OSX

Description

Tried calling str() on a caught exception and it blew up.

What I Did

>>> import fwffr                                                                                                                                                                                                

>>> str(fwffr.FixedLengthError("asdf"))                                                                                                                                                                         
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/diegoargueta/.pyenv/versions/3.7.6/envs/gds3/lib/python3.7/site-packages/fwffr.py", line 31, in __str__
    return self.message
AttributeError: 'FixedLengthError' object has no attribute 'message'

'FixedLengthError' object has no attribute 'message'

Either this should be an abstract base class or some tweaking is needed for rendering the error message.