Updated PowerShell wrapping how-to with Try/Catch block to fix failing silently

[coreypenford]

With the current recommended PowerShell wrapping, if it cannot run the script (due to a missing script or bad script i.e. misplaced characters/bad syntax), it returns 0 and fails silently. This is due to $lastexitcode returning a 0 when it is unable to even start to run a script.

Failing silently is never a good default for monitoring software, and this can be a big issue when mass deploying custom checks as it is impractical to manually verify each one.

This PR updates the how-to documentation to include a Try/Catch block on the PS wrappings. This way if powershell is unable to run the script, it exits with a 3 and reports the $error. Otherwise, the script will run and return it's $lastexitcode.

Testing and discussion occurred here originally. https://support.nagios.com/forum/viewtopic.php?f=6&t=51591

I looked if the default nsclient.ini included the wrapping and it did not, so it appears the documentation is the only thing that needs to be updated. While updating the documentation I fixed some wording and a few typos as well, hope you don't mind. Feel free to make any changes you'd like.