Output more details when "plugins update" command fails

Athena version: fb66cf62052381827af6e7cc0395b976cbb481cb

When running this command inside my own fork:

```sh
./athena plugins update php --athena-no-logo
```

I got the following error output (not very informative):

```
[ERROR] there was a problem updating the plugin 'php'
```

It would be handy to have the `git` command `stderr` contents appended to our error message.