handle nil pointer for saml user session validation

[irshadaj]

Description

When a User is updated to be SAML, its AuthSecret is set to nil. Thereafter in ValidateSessions, we call a method on the nil AuthSecret leading to a panic in the logs. This MR adds logic to handle that case gracefully.

I also updated the Auth Middleware to return the actual error upon ValidateSession failure, as we were just discarding this information and returning a blanket failure statement before.

How Has This Been Tested?

Manual testing

Types of changes

• Chore (a change that does not modify the application functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Documentation updates are needed, and have been made accordingly.
• I have added and/or updated tests to cover my changes.
• All new and existing tests passed.
• My changes include a database migration.