Added scope to Update-Module. Added localized error message

src/PowerShellGet/PSGet.Resource.psd1

@@ -22,6 +22,7 @@ ConvertFrom-StringData @'
         RequiredVersionAllowedOnlyWithSingleModuleName=The RequiredVersion parameter is allowed only when a single module name is specified as the value of the Name parameter, without any wildcard characters.
         MinimumVersionIsGreaterThanMaximumVersion=The specified MinimumVersion '{0}' is greater than the specified MaximumVersion '{1}'.
         AllowPrereleaseRequiredToUsePrereleaseStringInVersion=The '-AllowPrerelease' parameter must be specified when using the Prerelease string in MinimumVersion, MaximumVersion, or RequiredVersion.
+        UpdateModuleAdminPrivilegeRequiredForAllUsersScope=Administrator rights are required to update modules in '{0}'. Log on to the computer with an account that has Administrator rights, and then try again, or update '{1}' by adding "-Scope CurrentUser" to your command. You can also try running the Windows PowerShell session with elevated rights (Run as Administrator).
         InstallModuleAdminPrivilegeRequiredForAllUsersScope=Administrator rights are required to install modules in '{0}'. Log on to the computer with an account that has Administrator rights, and then try again, or install '{1}' by adding "-Scope CurrentUser" to your command. You can also try running the Windows PowerShell session with elevated rights (Run as Administrator).
         InstallScriptAdminPrivilegeRequiredForAllUsersScope=Administrator rights are required to install scripts in '{0}'. Log on to the computer with an account that has Administrator rights, and then try again, or install '{1}' by adding "-Scope CurrentUser" to your command. You can also try running the Windows PowerShell session with elevated rights (Run as Administrator).
         AdministratorRightsNeededOrSpecifyCurrentUserScope=Administrator rights are required to install or update. Log on to the computer with an account that has Administrator rights, and then try again, or install by adding "-Scope CurrentUser" to your command. You can also try running the Windows PowerShell session with elevated rights (Run as Administrator).

src/PowerShellGet/public/psgetfunctions/Update-Module.ps1

@@ -26,6 +26,11 @@ function Update-Module {
         [PSCredential]
         $Credential,
 
+        [Parameter()]
+        [ValidateSet("CurrentUser", "AllUsers")]
+        [string]
+        $Scope,
+
         [Parameter(ValueFromPipelineByPropertyName = $true)]
         [ValidateNotNullOrEmpty()]
         [Uri]
@@ -51,6 +56,17 @@ function Update-Module {
     Begin {
         Install-NuGetClientBinaries -CallerPSCmdlet $PSCmdlet -Proxy $Proxy -ProxyCredential $ProxyCredential
 
+        if ($Scope -eq "AllUsers" -and -not (Test-RunningAsElevated)) {
+          # Throw an error when Update-Module is used as a non-admin user and '-Scope AllUsers'
+          $message = $LocalizedData.UpdateModuleAdminPrivilegeRequiredForAllUsersScope -f @($script:programFilesModulesPath, $script:MyDocumentsModulesPath)
+
+          ThrowError -ExceptionName "System.ArgumentException" `
+              -ExceptionMessage $message `
+              -ErrorId "UpdateModuleAdminPrivilegeRequiredForAllUsersScope" `
+              -CallerPSCmdlet $PSCmdlet `
+              -ErrorCategory InvalidArgument
+        }
+
         # Module names already tried in the current pipeline
         $moduleNamesInPipeline = @()
     }
@@ -135,7 +151,11 @@ function Update-Module {
             $PSBoundParameters["PackageManagementProvider"] = $providerName
             $PSBoundParameters["InstallUpdate"] = $true
 
-            $PSBoundParameters["Scope"] = Get-InstallationScope -PreviousInstallLocation $psgetItemInfo.InstalledLocation -CurrentUserPath $script:MyDocumentsModulesPath
+            if (-not $Scope) {
+              $Scope = Get-InstallationScope -PreviousInstallLocation $psgetItemInfo.InstalledLocation -CurrentUserPath $script:MyDocumentsModulesPath
+            }
+
+            $PSBoundParameters["Scope"] = $Scope
 
             $sid = PackageManagement\Install-Package @PSBoundParameters
         }