Wilken Rivera 35df3914d2 provisioner/powershell: Update default execute command to handle script errors
This change sets the ErrorActionPreference and wraps the script execution in a Try/Catch statement so that the provisioner can capture any errors encountered when running the script. In addition to the try/catch the `&` operator is replaced by the `.` sourcing operator to ensure the script is executed in the same scope as the parent command (so that errors bubble up properly).

Tests after change
```
⇶  ACC_TEST_BUILDERS=amazon-ebs ACC_TEST_PROVISIONERS=powershell go test ./provisioner/powershell/... -timeout=1h
ok      github.com/hashicorp/packer/provisioner/powershell      915.865s
```
2020-05-21 09:05:43 -04:00
..