fix DEBUG=1

the errors.py code was overwriting the excepthook installed in qt.py

aqt/errors.py

@@ -8,11 +8,12 @@ from anki.lang import _
 from aqt.qt import *
 from aqt.utils import showText, showWarning
 
-def excepthook(etype,val,tb):
+if not os.environ.get("DEBUG"):
+    def excepthook(etype,val,tb):
         sys.stderr.write("Caught exception:\n%s%s\n" % (
             ''.join(traceback.format_tb(tb)),
             '{0}: {1}'.format(etype, val)))
-sys.excepthook = excepthook
+    sys.excepthook = excepthook
 
 class ErrorHandler(QObject):
     "Catch stderr and write into buffer."