Replace hardwired error handlers in tui_initialize_io

Replace hardwired error handlers in tui_initialize_io

tui_initialize_io contains a pair of hardwired fprintf/exit error
handlers.  I was unable to find any documentation as to why they're
hardwired (the code appeared in a monolithic block back in 2001:
https://sourceware.org/ml/gdb-patches/2001-07/msg00490.html) and I
was also unable to come up with a situation where error would not
be suitable, so I have replaced both handlers with calls to error.

gdb/ChangeLog:

	* tui/tui-io.c (tui_initialize_io): Replace two fprintf/exit
	pairs with calls to error.  Wrap the message with _().