Stop Get-Process from adding exceptions to $Error

Updating the Get-Process call in GitUtils.ps1 so exceptions aren't added to the global $Error array

This occurs when pageant or ssh-agent aren't running, and therefore there is no process / pid to retrieve. However this is a valid scenario so shouldn't be added to the global $Error array.

Also being more explicit with the check of the returned value from Get-Process
Revert "Stop Get-Process from adding exceptions to $Error"

This reverts commit 22d1fc48fa78bb14ed22f024400c9e64ef8c3c34.
Modifying the identification of the pageant and ssh-agent processes

Changing the identification of the pageant / ssh-agent process so an
error being thrown and silently ignored is not an expected scenario and
allows real errors from calling Get-Process to be seen
Removing the unneeded selection of ssh-agent's PID
Merge pull request #162 from paulmarsy/master

Stop Get-Process from adding exceptions to $Error when checking if pageant / ssh-agent aren't running