Describe specific exceptions.

[fuzzball81]

Add git describe specific exceptions, extracting details for the failure from GitPython

Description

If there is a failure during a git describe operation a return code of 128 is always used and all the details are included in failure text. Instead of using a general exception, add a more specific describe exception and extract the failure text for GitPython and add it to the exception.

It may be a good idea to have several types of exceptions based on the common failure modes for more granular error handling but that can be another enhancement in the future.

[jpichon]

The specific failure message should already be available in the generic describe exception added with PR23 (sorry for the confusion!).

An issue with simply using the failure text to determine a specific exception is that the message comes from git and can be localised. For example, calling with a bad hash:

$ git describe kfoejoe
fatal: Not a valid object name kfoejoe
$ git describe ifihe
fatal: ifihe n'est pas un nom d'objet valide

So we may have to figure out what caused the most common errors by other means.