Show nice message when git cannot be found

[JakeGinnivan]

INFO [07/28/15 11:14:12:11] Could not execute 'git log' due to the following error:
System.IO.FileNotFoundException: The executable file 'git' could not be found.
File name: 'git' ---> System.ComponentModel.Win32Exception (0x80004005): The system cannot find the file specified

We should show something like:

Unable to display git log, this is just for debugging purposes to give more information to track down your issue. Run gitversion debug locally instead.

Relies on #421 to be finished for the gitversion debug message. Can show git log --graph --format=""%h %cr %d"" --decorate --date=relative --all --remotes=* instead for the moment.