connector/saml: make unparsable (trailing, non-space/newline) data an error

Fixes #1304, if we want to be harsh.

However, I think if it was the user's intention to pass two certs, and
the second one couldn't be read, that shouldn't just disappear. After
all, when attempting to login later, that might fail because the
expected IdP cert data isn't there.

Signed-off-by: Stephan Renatus <srenatus@chef.io>