`cbt ls` error message is obtuse when using a bad instance ID

If I run `cbt -instance=abc_123 ls` I get an error message which does not help me diagnose my problem. I get:

```
$ cbt -instance=abc_123 ls
2023/05/02 15:18:54 -creds flag unset, will use gcloud credential
2023/05/02 15:18:54 -project flag unset, will use gcloud active project
2023/05/02 15:18:54 gcloud active project is "autonomous-mote-782"
2023/05/02 15:18:54 Getting list of tables: rpc error: code = InvalidArgument desc = When parsing 'projects/autonomous-mote-782/instances/abc_123' : Instance name expected in the form 'projects//instances/'.
error details: name = DebugInfo detail = [ORIGINAL ERROR] generic::invalid_argument: When parsing 'projects/autonomous-mote-782/instances/abc_123' : Instance name expected in the form 'projects//instances/'. [google.rpc.error_details_ext] { message: "When parsing \'projects/autonomous-mote-782/instances/abc_123\' : Instance name expected in the form \'projects//instances/\'." } stack =
```

"Instance name expected in the form \'projects//instances/\'" is very confusing since my instance name seemingly does match this pattern.

Conversely, if I do `cbt -instance=abc ls`, I get a better error message explaining what is wrong:

```
$ cbt -instance=abc ls
2023/05/02 15:18:46 -creds flag unset, will use gcloud credential
2023/05/02 15:18:46 -project flag unset, will use gcloud active project
2023/05/02 15:18:47 gcloud active project is "autonomous-mote-782"
2023/05/02 15:18:47 Getting list of tables: rpc error: code = InvalidArgument desc = When parsing 'projects/autonomous-mote-782/instances/abc' : Invalid id for collection instances : Length should be between [6,33], but found 3 'abc'
error details: name = DebugInfo detail = [ORIGINAL ERROR] generic::invalid_argument: When parsing 'projects/autonomous-mote-782/instances/abc' : Invalid id for collection instances : Length should be between [6,33], but found 3 'abc' [google.rpc.error_details_ext] { message: "When parsing \'projects/autonomous-mote-782/instances/abc\' : Invalid id for collection instances : Length should be between [6,33], but found 3 \'abc\'" } stack =
```

It would be nice if the CLI told me why the instance name was unparseable or at least show me the regex which my instance ID needs to match.