contrib: add a hint if the Windows dependency MSI is already installed

In the past, I've gotten confused when the script failed on seemingly random
python installs (and thus the py3.8 install was commented out from the last time
this happened to me, which has been reverted here).  This particular error code
means the package was already installed.  For python, it means the major and
minor version are the same, but the micro version may differ.

In practice, ignoring the python installation failure will cause the pip
installation that happens next to fail, because python.exe for that version is
somewhere else on the system.  This could probably be fixed by running py.exe
with the major and minor version, but that is skipped during the install for
some reason.  I didn't feel like over complicating this though, and at least
there's a better hint when the problem occurs.

Differential Revision: https://phab.mercurial-scm.org/D12560

contrib/install-windows-dependencies.ps1

@@ -90,9 +90,15 b' function Invoke-Process($path, $argument'
     $p = Start-Process -FilePath $path -ArgumentList $arguments -Wait -PassThru -WindowStyle Hidden
 
     if ($p.ExitCode -ne 0) {
+        # If the MSI is already installed, ignore the error
+        if ($p.ExitCode -eq 1638) {
+            Write-Output "program already installed; continuing..."
+        }
+        else {
         throw "process exited non-0: $($p.ExitCode)"
     }
 }
+}
 
 function Install-Python3($name, $installer, $dest, $pip) {
     Write-Output "installing $name"
@@ -150,7 +156,7 b' function Install-Dependencies($prefix) {'
     Install-Python3 "Python 3.7 32-bit" ${prefix}\assets\python37-x86.exe ${prefix}\python37-x86 ${pip}
     Install-Python3 "Python 3.7 64-bit" ${prefix}\assets\python37-x64.exe ${prefix}\python37-x64 ${pip}
     Install-Python3 "Python 3.8 32-bit" ${prefix}\assets\python38-x86.exe ${prefix}\python38-x86 ${pip}
-#    Install-Python3 "Python 3.8 64-bit" ${prefix}\assets\python38-x64.exe ${prefix}\python38-x64 ${pip}
+    Install-Python3 "Python 3.8 64-bit" ${prefix}\assets\python38-x64.exe ${prefix}\python38-x64 ${pip}
     Install-Python3 "Python 3.9 32-bit" ${prefix}\assets\python39-x86.exe ${prefix}\python39-x86 ${pip}
     Install-Python3 "Python 3.9 64-bit" ${prefix}\assets\python39-x64.exe ${prefix}\python39-x64 ${pip}
     Install-Python3 "Python 3.10 32-bit" ${prefix}\assets\python310-x86.exe ${prefix}\python310-x86 ${pip}