Credential Manager: Remove WebString from WebCredentialManagerError.

The enum gives enough detail; we don't need an additional string to be
created and passed around.

BUG=400674

Review URL: https://codereview.chromium.org/618723002

git-svn-id: svn://svn.chromium.org/blink/trunk@183056 bbb929c8-8fbe-4397-9dbb-9b2b20218538