Change tui_source_window_base::set_contents to return bool

This changes tui_source_window_base::set_contents to return bool,
rather than tui_status.  It also changes one implementation of
set_contents to use early returns rather than a variable, which IMO
makes it easier to follow.

gdb/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (struct tui_source_window_base)
	<set_contents>: Return bool.
	* tui/tui-winsource.c
	(tui_source_window_base::update_source_window_as_is): Update.
	* tui/tui-source.h (struct tui_source_window) <set_contents>:
	Return bool.
	* tui/tui-source.c (tui_source_window::set_contents): Return
	bool.  Simplify.
	* tui/tui-disasm.h (struct tui_disasm_window) <set_contents>:
	Return bool.
	* tui/tui-disasm.c (tui_disasm_window::set_contents): Return
	bool.

Change-Id: I8c5212400cd7aadf35760c22d5344cd3b9435674