format errors from auth-jwt plugin like other authentication errors

[meirish]

When the OIDC flow errors, what the UI displayed wasn't particularly helpful - this PR changes it so that errors from the auth-jwt component get passed through the same path as errors in the auth-form components instead of directly setting auth-form's error property.

Before:

After

[madalynrose]

This looks great! Is there an instance where we wouldn't want to format the error this way?

[meirish]

@madalynrose The default is to preface it with Authentication failed: and then join the errors. I think since we're setting errorMessage in the message-error component, we'll always want the error to be a string (which is what the handleError method does.