Fix SAML user does not exist logging call

[jpmanthorp]

Caused by how we did not use Python's old string formatting operator to inject the problematic id into the log message. This issue caused one of our SAML logging calls to raise its own error last week and consequently the "user not found error" percolated up to a user instead of redirecting them to a denied page. For the sake of clarity, we will now manually format the string injected into the logger (as opposed to allowing the logging module to only generate the final msg if the log would actually be produced) as this is a fairly inexpensive operation.

See logs here