[general] Handle Errors/Exceptions consistently.

[mkustermann]

We need to discuss how to handle errors consistently:

• what errors to let through from lower levels
• what errors to wrap into new errors
• how many exception classes we want (and how many built-in exception classes we reuse)
• a sensible exception/error class hierarchy

[pmagnuson]

I second this. I recently started using gcloud in Flutter. I have found that errors are not trapped successfully using a try {} catch {} block.

try {
      auth.clientViaServiceAccount(_credentials, _SCOPES).then((client) {
        final storage = Storage(client, 'project');
        final b = storage.bucket('bucket');
        b.read('object`).pipe(outFile.openWrite()).catchError((e) {
          print('error ${e.toString()}');
        }).whenComplete(() {
          print('file download complete');
          setState(() {
            status = Status.complete;
          });
        });
      });
    } catch (e) {
      print('error = ${e.toString()}');
      setState(() {
        status = Status.error;
      });
    }

neither the catchError() or the catch {} block are invoked if the project, bucket, or object are incorrect. Also in the whenComplete() I have found no way to check if the download was successful.