[spu] throw error when target_read_memory fails

[spu] throw error when target_read_memory fails

I happen to see that 1 is returned in spu_software_single_step when
target_read_memory returns 1.  It must be wrong.  That patch changes
it to throwing an error.  Note that I choose to throw error because I
find the code in the end of spu_software_single_step throws errors.

gdb:

2016-03-18  Yao Qi  <yao.qi@linaro.org>

	* spu-tdep.c (spu_software_single_step): Throw error when
	target_read_memory fails.