Various Powershell Fixes (#7854)

* Remove incorrect x64 error message when showing usage description

* Add missing escape characters in usage description

The actual environment variable value would be printed without these
escape characters.

* Add missing quotation marks in usage description

* Also test existence of sha and signature files

For multiple reason the executable could be downloaded, but not the
signature files. Later on the script will error out because these files
are lacking.

Co-authored-by: Raul Jordan <raul@prysmaticlabs.com>

prysm.ps1

@@ -3,7 +3,6 @@ $ProgressPreference = 'SilentlyContinue' # Disable Invoke-WebRequest progress ba
 
 # Complain if invalid arguments were provided.
 if ("beacon-chain", "validator", "slasher" -notcontains $args[0]) {
-    Write-Host "ERROR: prysm is only supported on 64-bit Operating Systems" -ForegroundColor Red;
     Write-Host @"
 Usage: ./prysm.sh1 PROCESS FLAGS.
 
@@ -14,12 +13,12 @@ Use this script to download the latest Prysm release binaries.
 Downloaded binaries are saved to .\dist
  
 To specify a specific release version:
-  $env:USE_PRYSM_VERSION=v1.0.0-alpha3
+  `$env:USE_PRYSM_VERSION="v1.0.0-beta.3"
  to resume using the latest release:
   Remove-Item env:USE_PRYSM_VERSION
  
 To automatically restart crashed processes:
-  $env:PRYSM_AUTORESTART=$TRUE ; .\prysm.sh1 beacon-chain
+  `$env:PRYSM_AUTORESTART=`$TRUE ; .\prysm.sh1 beacon-chain
  to stop autorestart run:
   Remove-Item env:PRYSM_AUTORESTART
 "@;
@@ -61,7 +60,7 @@ else {
 $fileName = "$($args[0])-$version-windows-amd64.exe";
 $folderBin = "$folderDist\$fileName";
 
-if (Test-Path $folderBin) {
+if ((Test-Path $folderBin) -and (Test-Path "$folderBin.sha256") -and (Test-Path "$folderBin.sig")) {
     Write-Host "$($args[0]) is up to date with version: $version" -ForegroundColor Green;
 }
 else {