Fix nil dereference when core uninstall fails during upgrade

[matthijskooijman]

The code tried to log the wrong error, which was nil, and resulted in a
nil dereference.

This would occur 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, uninstalling the previous version
would fail (not managed by package manager) and the nil dereference
would happen:

$ 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@1.8.3 with arduino:avr@1.8.2...
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0xde10dc]

goroutine 1 [running]:
github.com/arduino/arduino-cli/commands/core.installPlatform(0xc00038b600, 0xc0002406e0, 0xc0002ea560, 0x3, 0x4, 0xc0002ea540, 0xc0015c11d0, 0x0, 0x58, 0x68)
    /home/matthijs/docs/Electronics/Arduino/arduino-cli/commands/core/install.go:148 +0x10fc
github.com/arduino/arduino-cli/commands/core.PlatformInstall(0x1203c00, 0xc0000360f0, 0xc0000aef50, 0xc0002ea540, 0xc0015c11d0, 0x1, 0x1, 0x0)
    /home/matthijs/docs/Electronics/Arduino/arduino-cli/commands/core/install.go:52 +0x2ce
github.com/arduino/arduino-cli/cli/core.runInstallCommand(0xc000422840, 0xc0003b1a10, 0x1, 0x1)
    /home/matthijs/docs/Electronics/Arduino/arduino-cli/cli/core/install.go:103 +0x2a5
github.com/spf13/cobra.(*Command).execute(0xc000422840, 0xc0003b19f0, 0x1, 0x1, 0xc000422840, 0xc0003b19f0)
    /home/matthijs/docs/src/go/pkg/mod/github.com/spf13/cobra@v1.0.1-0.20200710201246-675ae5f5a98c/command.go:846 +0x2c2
github.com/spf13/cobra.(*Command).ExecuteC(0xc000322dc0, 0x3a, 0xc00037f680, 0xc0001f6a80)
    /home/matthijs/docs/src/go/pkg/mod/github.com/spf13/cobra@v1.0.1-0.20200710201246-675ae5f5a98c/command.go:950 +0x375
github.com/spf13/cobra.(*Command).Execute(...)
    /home/matthijs/docs/src/go/pkg/mod/github.com/spf13/cobra@v1.0.1-0.20200710201246-675ae5f5a98c/command.go:887
main.main()
    /home/matthijs/docs/Electronics/Arduino/arduino-cli/main.go:31 +0x8a

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@1.8.3 with arduino:avr@1.8.2...
Error upgrading platform: arduino:avr@1.8.3 is not managed by package manager...
Error during install: upgrading platform: arduino:avr@1.8.3 is not managed by package manager

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features) Sorry, that would need quite a bit of work for such a trivial fix, no time for that.
• Docs have been added / updated (for bug fixes / features) Not applicable
• UPGRADING.md has been updated with a migration guide (for breaking changes) Not applicable

[cmaglie]

@matthijskooijman can you rebase this one?

[matthijskooijman]

@cmaglie, Done. Turns out some other change already fixed the nil-dereference and reduced it to printing %!s(<nil>, which is still not ideal. I've updated this PR to fix that and properly print the error, and updated the commit message as well.

[cmaglie]

I cannot see it, did you forget to push?
(since you're there if you can rebase again on the current master will also fix a problem in the test runners)

[matthijskooijman]

w00ps, I pushed to this repo, rather than my fork accidentally :-)

I rebased again (I didn't retest, but it seems unlikely that the latest changes would break this), and pushed to the right repo this time (and cleaned up the accidentally pushed branch too).

[matthijskooijman]

Hm, seems there is still some problem with codecov in the tests: https://github.com/matthijskooijman/arduino-cli/runs/3300616293?check_suite_focus=true

[silvanocerza]

Hm, seems there is still some problem with codecov in the tests: https://github.com/matthijskooijman/arduino-cli/runs/3300616293?check_suite_focus=true

That's normal, you're missing the credentials secrets to push from your fork. Don't worry about it, if the PR checks are passing it's fine.

There was a spurious integration tests failure but I restarted the workflows, hope it doesn't fail again.