Adding try/catch logic to Write-VcsStatus

Adding try/catch logic to Write-VcsStatus so that if one of the scriptblocks that has been added to the VcsPromptStatuses array fails the other scriptblocks are still invoked and the effect on the user's prompt is minimized

Use of the ErrorVariable parameter during invocation is to also capture any errors that were written to the error stream

GitPrompt.ps1

@@ -196,6 +196,6 @@ function Global:Write-VcsStatus {
 $PoshGitVcsPrompt = {
     $Global:GitStatus = Get-GitStatus
     Write-GitStatus $GitStatus
-}
+}
 $Global:VcsPromptStatuses += $PoshGitVcsPrompt
 $ExecutionContext.SessionState.Module.OnRemove = { $Global:VcsPromptStatuses = $Global:VcsPromptStatuses | ? { $_ -ne $PoshGitVcsPrompt} }