Error messages not printed properly

[carolynvs]

When I try to read secrets from a vault that I forgot to setup an Access Policy for the service principal, it causes an error. However the error is not printed in the proper plugin format so you can't tell what's wrong

$ porter install -c myclaim --tag carolynvs/myapp:v0.0.3
installing myapp...
Error: could not load credentials: 3 errors occurred:
	* unable to resolve credential myapp.azure_client_id from secret myapp-client-id: could not connect to the secrets.porter.carolynvs plugin: Unrecognized remote plugin message:

This usually means that the plugin is either invalid or simply
needs to be recompiled to support the latest protocol.

[carolynvs]

We should reverify with this change

getporter/porter#1028