[processing] Always log python exceptions to logger

Because Python code cannot catch and rethrow c++ exceptions
without losing context, we end up getting unknown exceptions
caught by c++ from python code (phew!).

So if we catch a python exception from processing, throw it
into the log so at least there's the full debugging detail
available on the console.

(Not ideal. Would love to see a PR allowing the full error
message to be passed correctly between c++/python/back again)

(cherry-picked from f4445a0)

Author: nyalldawson

python/core/core.sip.in

@@ -6,6 +6,7 @@ ${DEFAULTDOCSTRINGSIGNATURE}
 %ModuleCode
 
 #include "qgsexception.h"
+#include "qgslogger.h"
 
 QString getTraceback()
 {
@@ -107,6 +108,7 @@ done:
 
 %VirtualErrorHandler processing_exception_handler
     QString trace = getTraceback();
+    QgsLogger::critical( trace );
     SIP_RELEASE_GIL( sipGILState );
     throw QgsProcessingException( trace );
 %End