Indicate batch mode failures by exiting with nonzero status

This commit causes GDB in batch mode to exit with nonzero status
if the last command to be executed fails.

gdb/ChangeLog:

	PR gdb/13000:
	* gdb/main.c (captured_main_1): Exit with nonzero status
	in batch mode if the last command to be executed failed.
	* NEWS: Mention the above.

gdb/testsuite/ChangeLog:

	PR gdb/13000:
	* gdb.base/batch-exit-status.exp: New file.
	* gdb.base/batch-exit-status.good-commands: Likewise.
	* gdb.base/batch-exit-status.bad-commands: Likewise.