Change routine of error return; Protect around thread.

1. Instead of returning error code right after it occurs, use goto to put error handling together.
2. get_task_struct(thread) to protect thread coloring.

	modified:   kernel/color.c
Add usage message to test program.

	modified:   test/forktest.c
	modified:   test/getcolors.c
	modified:   test/setcolors.c