Documentation: --log-level=verbose causes web UI to exit with no message

Documentation suggests to use log-level VERBOSE:

https://github.com/search?q=repo%3AGoogleCloudPlatform%2Fspanner-migration-tool+%2Flog+level+.*+verbose%2F&type=code

But running the following command causes the tool to exit with no log message and status 0 - success
```
gcloud alpha spanner migrate web --log-level=debug --log-level=verbose
```

This may be because the zap log library does not support a "verbose" level, and it should be "debug", the log library fails to initialize, but the error is sent to the log library to be reported...

https://github.com/GoogleCloudPlatform/spanner-migration-tool/blob/5f58718589dd96668d52b215b3df577b818cdba2/webv2/webCmd.go#L75