Fix session cookie error

[geoff-va]

Session cookie error still not fixed!

This is where the error is raised:

• https://github.com/dmitry-viskov/pylti1.3/blob/0dd9a52d75110cdab5797cf23f6678c41c2f3124/pylti1p3/message_launch.py?plain=1#L758

The actual error text that seems to come through is Missing %s cookie session-id - like the % substitution isn't working.

Maybe can actually solve this via error reformatters here