traceback: Add basic versions of format_exc() and format_exception().

traceback/traceback.py

@@ -6,6 +6,9 @@ def format_tb(tb, limit):
 def format_exception_only(type, value):
     return [repr(value) + "\n"]
 
+def format_exception(etype, value, tb, limit=None, chain=True):
+    return format_exception_only(etype, value)
+
 def print_exception(t, e, tb, limit=None, file=None, chain=True):
     if file is None:
         file = sys.stdout
@@ -13,3 +16,6 @@ def print_exception(t, e, tb, limit=None, file=None, chain=True):
 
 def print_exc(limit=None, file=None, chain=True):
     print_exception(*sys.exc_info(), limit=limit, file=file, chain=chain)
+
+def format_exc(limit=None, chain=True):
+    return "".join(format_exception(*sys.exc_info(), limit=limit, chain=chain))