Error logging improvement.

- Implemented unified function for error handling.
- Each error is assigned its own identifier, generated by using the first
8 chars of the $request_id variable.
- Added support for JSON log output. This is controlled by the
$oidc_log_format variable, which must be set to 'json'.
- Added support for stack trace output. This is enabled by the
$oidc_debug variable, which must have any non-empty value.
If this variable is defined, the $internal_error_message variable
is overwritten with the text of the last error and returned to
the User Agent - so use this only for debugging!

Author: route443