obscure error if jwtSecret isn't configured

[pekeler]

I think it would be good if this app threw an explicit error telling me that this value is missing, instead of this obscure stack trace: arangodb/arangodb#1696

[pluma]

Indeed. The problem is that the secret is not required because the "none" type encryption must not use a secret whereas every other type must use a secret. Therefore we can't rely on the app not being mounted unless properly configured and instead need to catch misconfiguration at runtime.