(GH-1000) Don't check $LASTEXITCODE by default

Starting with 33b6285, which was released as part of GH-512 in
0.9.10, we started checking `$LASTEXITCODE` in addition to the script
command success. This meant it offered the ability to capture when a
script exited with `exit 1` and handle that accordingly. However that
was not a recommended scenario for returning errors from scripts.

Checking `$LastExitCode` checks the last executable's exit code when the
script specifically does not call `Exit`. This can lead to very
perplexing failures, such as running a successful xcopy that exits with
2 and seeing package failures without understanding why. Since it is
not typically recommended to call `Exit` to return a value from
PowerShell because of issues with different hosts, it's less of a
concern to only look at explicit failures.

src/chocolatey.resources/helpers/chocolateyScriptRunner.ps1

@@ -46,10 +46,22 @@ $checksumExe = Join-Path $chocoTools 'checksum.exe'
 
 Write-Debug "Running `'$packageScript`'";
 & "$packageScript"
-
 $scriptSuccess = $?
+$lastExecutableExitCode = $LASTEXITCODE
+
+if ($lastExecutableExitCode -ne $null -and $lastExecutableExitCode -ne '') {
+  Write-Debug "The last executable that ran had an exit code of '$lastExecutableExitCode'."
+} 
+
+if (-not $scriptSuccess) {
+ Write-Debug "The script exited with a failure."
+} 
+
+$exitCode = 0
+if ($env:ChocolateyCheckLastExitCode -ne $null -and $env:ChocolateyCheckLastExitCode -eq 'true' -and $lastExecutableExitCode -ne $null -and $lastExecutableExitCode -ne '') {
+  $exitCode = $lastExecutableExitCode
+}
 
-$exitCode = $LASTEXITCODE
 if ($exitCode -eq 0 -and -not $scriptSuccess) {
   $exitCode = 1
 }