Fix bogus error message with error code from 7za amiss

Running choco install in a cmd window, spew out this error message:

```
DEBUG: wrapping 7za invocation with Write-FileUpdateLog
DEBUG: Running 'Write-FileUpdateLog' with logFilePath:'C:\Chocolatey\lib\scala.portable.2.10.4\scala.portableInstall.zip.txt'',
locationToMonitor:C:\Tools, Operation: '
    param($7zip, $destination, $fileFullPath, [ref]$exitCodeRef)
    $p = Start-Process $7zip -ArgumentList "x -o`"$destination`" -y `"$fileFullPath`"" -Wait -WindowStyle Hidden -PassThru
    $exitCodeRef.Value = $p.ExitCode
  '
DEBUG: Tracking current state of 'C:\Tools'
DEBUG: 7za exit code:
DEBUG: Renaming 'C:\Users\Claudio\AppData\Local\Temp\chocolatey\scala.portable\success.log' to
'C:\Users\Claudio\AppData\Local\Temp\chocolatey\scala.portable\success.log.old'
Write-Error : scala.portable did not finish successfully. Boo to the chocolatey gods!
-----------------------
[ERROR] 7-Zip signalled an unknown error (code )
-----------------------
At C:\Chocolatey\chocolateyInstall\helpers\functions\Write-ChocolateyFailure.ps1:30 char:3
+   Write-Error $errorMessage
+   ~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,Write-Error
```

Oddly enough, the error code is not set. Looking in the update log, it
seems that 7za stopped halfway through or was just not finished yet.

Might be related to
http://social.technet.microsoft.com/forums/en-US/3ec5c7b5-4a36-4890-98c2-ae654993f173/powershell-script-startprocess-no-exitcode-when-run-in-sccm-tasksequence
(GH-516) Guard all Wait-Process if posh v3+

 - This adds Wait-Process to anywhere Start-Process is called with a guard that
this should only be done in PowerShell v3+.
(maint) formatting
(GH-256) cpack/cpush always return 0 exit code even w/errors

 - This fixes the process to return the non-zero exit code when errors occur
Merge branch 'avdv/wait-for-process' into stable

* avdv/wait-for-process:
  (GH-256) cpack/cpush always return 0 exit code even w/errors
  (maint) formatting
  (GH-516) Guard all Wait-Process if posh v3+
  Fix bogus error message with error code from 7za amiss