Fix double prompt

If an error is thrown while handling a target event (within
fetch_inferior_event), and, the interpreter is not async (but the
target is), then GDB prints the prompt twice.

One way to see that in action is throw a QUIT while in a pagination
prompt issued from within fetch_inferior_event (or one of its
callees).  E.g. from the test:

 ---Type <return> to continue, or q <return> to quit---
 ^CQuit
 (gdb) (gdb) p 1
 ^^^^^^^^^^^
 $1 = 1
 (gdb)

The issue is that inferior_event_handler swallows errors and notifies
the observers (the interpreters) about the command error, even if the
interpreter is forced sync while we're handling a nested event loop
(for execute_command).  The observers print a prompt, and then when we
get back to the top event loop, we print another (in
start_event_loop).

I see no reason the error should be swallowed here.  Just cancel the
execution related bits and let the error propagate to the top level
(start_event_loop), which re-enables stdin and notifies observers.

gdb/
2014-07-14  Pedro Alves  <palves@redhat.com>

	* inf-loop.c (inferior_event_handler): Use TRY_CATCH instead of
	catch_errors.  Don't re-enable stdin or notify observers where,
	and rethrow error.
	(fetch_inferior_event_wrapper): Delete.

gdb/testsuite/
2014-07-14  Pedro Alves  <palves@redhat.com>

	* gdb.base/double-prompt-target-event-error.c: New file.
	* gdb.base/double-prompt-target-event-error.exp: New file.