Fix error printing when core uninstall fails during upgrade (#1371)

The code tried to log the wrong error, which was nil, hiding the real
error.

This originally occurred for example when a version of the core was already
installed manually in the sketchbook, and you would try to install
another version using arduino-cli, though that has since been fixed
(arduino-cli no longer tries to uninstall cores from the sketchbook).

To still reproduce this problem, you can e.g. break the permissions of
an installed core:

    $ arduino-cli core install arduino:[email protected]
    (...)
    $ chmod a-w ~/.arduino15/packages/arduino/hardware/avr/1.8.1
    $ arduino-cli core install arduino:[email protected]
    Tool arduino:[email protected] already installed
    Tool arduino:[email protected] already installed
    Tool arduino:[email protected] already installed
    Downloading packages...
    arduino:[email protected] already downloaded
    Upgrading arduino:[email protected] with arduino:[email protected]...
    Error upgrading platform: %!s(<nil>)...
    Error during install: upgrading platform: removing platform files: unlinkat /home/matthijs/.arduino15/packages/arduino/hardware/avr/1.8.1/firmwares: permission denied

With this fix applied, this produces a proper error message:

    $ arduino-cli core install arduino:[email protected]
    Tool arduino:[email protected] already installed
    Tool arduino:[email protected] already installed
    Tool arduino:[email protected] already installed
    Downloading packages...
    arduino:[email protected] already downloaded
    Upgrading arduino:[email protected] with arduino:[email protected]...
    Error upgrading platform: removing platform files: unlinkat /home/matthijs/.arduino15/packages/arduino/hardware/avr/1.8.1/firmwares: permission denied...
    Error during install: upgrading platform: removing platform files: unlinkat /home/matthijs/.arduino15/packages/arduino/hardware/avr/1.8.1/firmwares: permission denied

Author: matthijskooijman

Diff for: commands/core/install.go

@@ -145,7 +145,7 @@ func installPlatform(pm *packagemanager.PackageManager,
 		// In case of error try to rollback
 		if errUn != nil {
 			log.WithError(errUn).Error("Error upgrading platform.")
-			taskCB(&rpc.TaskProgress{Message: fmt.Sprintf(tr("Error upgrading platform: %s"), err)})
+			taskCB(&rpc.TaskProgress{Message: fmt.Sprintf(tr("Error upgrading platform: %s"), errUn)})
 
 			// Rollback
 			if err := pm.UninstallPlatform(platformRelease); err != nil {