Several improvements to SimpleSAML_Error_Exception:

- Exception messages and backtraces are now decoupled, so that they can be logged independently.
- Backtraces are now logged with "debug" log level, and only in case the "debug" configuration option is set.
- A new log() method allows the exception itself to decide which log level to use. This can be used by exceptions overriding this method to change the log level accordingly.
- Add a new parameter to the format() method so that the formatted message is anonymized, safe for display in the browser.

Additionally, this resolves #281.