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/
2014-08-05  Gary Benson  <gbenson@redhat.com>

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

If the requested interpreter cannot be set captured_main reports the
error with a hardwired fprintf/exit pair.  A fprintf/exit pair on the
previous line was replaced with a call to error in March 2003
(https://sourceware.org/ml/gdb-patches/2003-03/msg00444.html) but I
found no documentation as to why this particular hardwired handler
was left untouched.  I was also unable to come up with a situation
where error would not be suitable, so I have replaced it with a call
to error.

gdb/
2014-08-05  Gary Benson  <gbenson@redhat.com>

	* main.c (captured_main): Replace a fprintf/exit
	pair with a call to error.  Wrap the message with _().
List of calls to exit
False positives and other rubbish
Space the list
Document
Documentation
Replace another
Documentation
Documentation