Add translated error info in data for error template. This is needed in order for theming error page with Twig.

- Bugfix: the modules/saml/www/proxy/invalid_session.php shouldn't call directly the error handler in sspmod_saml_IdP_SAML2. Instead, it should use the SimpleSAML_Auth_State::throwException() method to let it handle the exception appropriately (in this case, it should always return back to the requester).
- The standard specifies that a "urn:oasis:names:tc:SAML:2.0:status:NoSupportedIDP" or "urn:oasis:names:tc:SAML:2.0:status:NoAvailableIDP" second-level status code should be returned to the requester in case an error occurs. Add a couple of exceptions to represent both statuses, and use them to set the right status code in the response.
- We shouldn't ask the user to logout in case the IDPList does not offer an IdP we recognize, or in case the proxy enforces the use of an IdP ('idp' configuration option in the auth source) and such IdP is in the IDPList.
- Similarly, these two cases should also handled in case we are authenticating for the first time, not only when reauthenticating.

Some things, like logging of SAML messages or backtraces, are controlled with the 'debug' configuration option. However, it might be possible that we don't want one while we want the other, but that's impossible with just one option.

This commit allows us to configure debugging options independently, but groupping all of them together. This is particularly useful if we want to log backtraces to debug errors, for example, but we don't want to log SAML messages to keep the privacy of the users. This also allows us to get rid of the 'debug.validatexml' configuration option, and group it with other debug options.

This changes are backwards-compatible. Old and new configurations will work at the same time.

It's not very useful to log backtraces always as debug, since that implies getting all the log messages, while backtraces would still help debug a particular error.

The code was enforcing this option even if a configuration was passed as a parameter to the constructor. If there is something wrong with a configuration and we are passing it to the constructor, we should know if at least the 'baseurlpath' is correct, and if not, fix it somehow.

This bug was producing a default configuration without 'baseurlpath', when no configuration was passed. In that situation, only the default path (/simplesaml) was working correctly.

This way we can discern why we couldn't set a cookie, and act accordingly.

Add a couple new exceptions, both to signal configuration issues. One can be used to signal an error in a certain configuration, leaving to the user deciding whether it was critical or not, and what to do to deal with it. The other one indicates a configuration exception that we cannot recover from (i.e. missing files), and bootstraps a minimal configuration skeleton that we can use to go on momentarily.

Fix build. It was broken due to a chicken & egg problem, with SimpleSAML_Configuration raising creating a new SimpleSAML_Error_Exception that tries to load the configuration.

- 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.

Add a proper message to the SimpleSAML_Error_NotFound exception, instead of printing its parameters.

(being respectful with occurences that might change the behaviour, i.e. default database prefixes)

Move SimpleSAML_Utilities::selfURLNoQuery() to SimpleSAML\Utils\HTTP::getSelfURLNoQuery() and deprecate the former.

Move SimpleSAML_Utilities::selfURL() to SimpleSAML\Utils\HTTP::getSelfURL() and deprecate the former.

Move SimpleSAML_Utilities:::getBaseURL() to SimpleSAML\Utils\HTTP::getBaseURL() and deprecate the former.

Stop calling SimpleSAML_Session::getAttributes() in SimpleSAML_Error_Error class. Use an alternate way to try to fetch user's mail.