Indicate batch mode failures by exiting with nonzero status

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.