Allow wrapping exceptions in CreateFailed

[althonos]

Hi Will,
I made it so CreateFailed accepts an additional exc argument, as discussed in #144.

I also added a decorator classmethod to CreateFailed, so that it is easier to wrap any errors occuring in a filesystem __init__ method into a CreateFailed exception.

[coveralls]

Coverage increased (+0.0002%) to 99.951% when pulling a34807b on althonos:master into 3a691e0 on PyFilesystem:master.

[coveralls]

Coverage increased (+0.0002%) to 99.951% when pulling a34807b on althonos:master into 3a691e0 on PyFilesystem:master.

[coveralls]

Coverage increased (+0.0002%) to 99.951% when pulling 138ce35 on althonos:master into 3a691e0 on PyFilesystem:master.

[willmcgugan]

Thanks.