Add gdb_report_register_access_error command

Add gdb_report_register_access_error command

With this option enabled (it's disabled by default) errors accessing
registers are returned to gdb. Otherwise they are ignored and success is
reported to gdb. (This is the current behavior.)

We want this for RISC-V, but there's still some cleanup that needs to be
done before that can be upstreamed.

Signed-off-by: Tim Newsome <tim@sifive.com>
Change-Id: I7e56109ea52d18b780c14a07fb35f9e6e8979da4
Reviewed-on: http://openocd.zylin.com/4452
Reviewed-by: Steven Stallion <sstallion@gmail.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Tested-by: jenkins