Make "register_tap_listener()" return NULL on success and a "GString *"

Make "register_tap_listener()" return NULL on success and a "GString *"
referring to a GString containing an error message on failure, and don't
have it print anything on failure.

If it fails, have its Tethereal-tap callers print an error message
before exiting, and have its Ethereal callers pop up a dialog box with
the error (except in cases where the failure is guaranteed not to be the
user's fault, and where we exit, in which case we just print an error
message before we exit).  In all cases, the error message includes the
text of the GString.

Fix a scanf format string in the DCE RPC statistics Ethereal tap, so
that it properly skips the comma before the filter string.

Fix some Ethereal error messages not to say "tethereal".

svn path=/trunk/; revision=7542