provide more information when a plugin barfs an exception

This is still not very useful because the stack trace is in the
context of the re-raise by futures.Future, not within the thread
where it was originally raised.  I'm not sure whether it's possible
to fix this without using a different thread handling mechanism.

See also issue saschpe#6.

rapport/report.py

@@ -24,6 +24,7 @@
     import concurrent.futures as futures
 else:
     import futures
+import traceback
 
 import jinja2
 
@@ -91,7 +92,8 @@ def create_report(plugins, timeframe):
             except jinja2.TemplateSyntaxError as e:
                 print >>sys.stderr, "Syntax error in plugin {0} at {1} line {2}: {3}".format(plugin, e.name, e.lineno, e.message)                
             except Exception as e:
-                print >>sys.stderr, "Failed plugin {0}:{1}: {2}!".format(plugin, plugin.alias, e)
+                print >>sys.stderr, "Failed plugin {0}:{1}:".format(plugin, plugin.alias)
+                print >>sys.stderr, traceback.format_exc()
 
     results_dict = {"login": rapport.config.get("user", "login"),
                     "date": report_date_string,