gdb/tui: convert if/error to an assert

gdb/tui: convert if/error to an assert

While working on the previous commit, I realised that there was an
error in tui_set_focus_command that could never be triggered.

Since the big tui rewrite (adding dynamic layouts) it is no longer
true that there is a tui_win_info object for every window at all
times.  We now only create a tui_win_info object for a particular
window, when the window is part of the current layout.  As a result,
if we have a tui_win_info pointer, then the window must be visible
inside tui_set_focus_command (this function calls tui_enable as its
first action, which makes the current layout visible).

The gdb.tui/tui-focus.exp test script exercises this area of code, and
doesn't trigger the assert, nor do any of our other existing tui
tests.