be exception text for auth error

[SimonCropp]

Currently a auth failure results in

Error occurred: LibGit2Sharp.LibGit2SharpException: Request failed with status code: 401
   at LibGit2Sharp.Core.Ensure.HandleError(Int32 result) in :line 0
   at LibGit2Sharp.Core.Proxy.git_remote_fetch(RemoteSafeHandle remote, Signature signature, String logMessage) in :line 0
   at LibGit2Sharp.Network.DoFetch(RemoteSafeHandle remoteHandle, FetchOptions options, Signature signature, String logMessage) in :line 0
   at LibGit2Sharp.Network.Fetch(Remote remote, FetchOptions options, Signature signature, String logMessage) in :line 0
   at GitVersion.GitHelper.NormalizeGitDirectory(String gitDirectory, Arguments arguments, String branch) in c:\BuildAgent\work\5e0d79a4e3943b17\GitVersionCore\BuildServers\GitHelper.cs:line 17
   at GitVersion.TeamCity.PerformPreProcessingSteps(String gitDirectory) in c:\BuildAgent\work\5e0d79a4e3943b17\GitVersionCore\BuildServers\TeamCity.cs:line 26
   at GitVersionTask.VersionAndBranchFinder.TryGetVersion(String directory, SemanticVersion& versionAndBranch) in c:\BuildAgent\work\5e0d79a4e3943b17\GitVersionTask\VersionAndBranchFinder.cs:line 28
   at GitVersionTask.WriteVersionInfoToBuildLog.InnerExecute() in c:\BuildAgent\work\5e0d79a4e3943b17\GitVersionTask\WriteVersionInfoToBuildLog.cs:line 50
   at GitVersionTask.WriteVersionInfoToBuildLog.Execute() in c:\BuildAgent\work\5e0d79a4e3943b17\GitVersionTask\WriteVersionInfoToBuildLog.cs:line 28 

it would be better if the exception was caught and a more helpful exceptions was raised

[RaphHaddad]

I will submit a PR for this. Then continue on #504.