Handle configuration errors more gracefully

[ChristianSi]

In several instances, errors in the config file lead to an error being thrown in the code. It would be better to reserve error for programming mistakes and instead throwError $ ThentosConfigurationError msg in such cases. This concerns e.g. (but not only) the sendUserConfirmationMail and sendPasswordResetMail actions, where unknown context variables ({{...}}) in mail body templates lead to errors.