exc: Addcallingcontext adds frames, not names as error context

Fix this thinko in Addcallingcontext description.

In current Error.Error() only function names are printed but with actual
frames there in error context clients will be probably able to access
this information theirselves.

exc/error.go

@@ -148,7 +148,7 @@ func init() {
 }
 
 // add calling context to error.
-// Add calling function names as error context up-to topfunc not including.
+// Add calling function frames as error context up-to topfunc not including.
 // see also: Addcontext()
 func Addcallingcontext(topfunc string, e *Error) *Error {
 	seenraise := false