Fix error printing when core uninstall fails during upgrade

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:avr@1.8.1
    (...)
    $ chmod a-w ~/.arduino15/packages/arduino/hardware/avr/1.8.1
    $ arduino-cli core install arduino:avr@1.8.2
    Tool arduino:avr-gcc@7.3.0-atmel3.6.1-arduino5 already installed
    Tool arduino:avrdude@6.3.0-arduino17 already installed
    Tool arduino:arduinoOTA@1.3.0 already installed
    Downloading packages...
    arduino:avr@1.8.2 already downloaded
    Upgrading arduino:avr@1.8.1 with arduino:avr@1.8.2...
    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:avr@1.8.2
    Tool arduino:avr-gcc@7.3.0-atmel3.6.1-arduino5 already installed
    Tool arduino:avrdude@6.3.0-arduino17 already installed
    Tool arduino:arduinoOTA@1.3.0 already installed
    Downloading packages...
    arduino:avr@1.8.2 already downloaded
    Upgrading arduino:avr@1.8.1 with arduino:avr@1.8.2...
    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

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 {