signed AuthnRequest from entity not in metadata flagged as IncorrectlySigned

[rhoerbe]

misleading exception if condition mentioned in title occurs

Code Version

4.4.0

Expected Behavior

exception should name the correct cause

Possible Solution

(1) rename exception to "unkown Entity or invalid request signature", or (better)
(2) validate entityid against metadata before validating signature

Steps to Reproduce

• remove the message obfuscation with rerasing exceptions as SATOSAUnknownError in base.run()
• issue request from unregistered entity

[peppelinux]

Also related to
#227