(#142) Adding try/catches

- In an attempt to correctly report an error in psake script, adding try/catch blocks
- Also, use the psake variable to return the correct exit code
(#142) Adding artifacts to appveyor.yml file

- So that we can take a look, if required, at the generated files from the build, let's add them as artifacts.
(#142) Changing the way GitVersion is called

Based on suggestion here:

https://github.com/Particular/GitVersion/issues/254

Using ```/output buildserver``` to force download of all branches.

Having to call it twice, as I now need to call it once to force download of branches, and another time to get access to the LegacySemVer variable.