Rephrase error log lines for AuthnFailed responses

Currently, when an IdP responds with an AuthnFailed message without an additional StatusMessage or StatusDetails, the SP writes the following line to the error log:

SAML response contained an error

This is slightly misleading in that it suggests that the SAML response XML message itself is not syntactically/semantically valid, as opposed to merely reporting that the IdP returned an error. Perhaps this error log message could be rewritten to better make the distinction. Something like:

SAML response reported an IdP error: (no message)
SAML response reported an IdP error: <StatusMessage> goes here