Fix error propagation from the Z0 packet in gdb-remote breakpoint setting.

The error was not getting propagated to the caller, so the higher layers thought the breakpoint
was successfully set & resolved.

I added a testcase, but it assumes 0x0 is not a valid place to set a breakpoint.  On most systems
that is true, but if it isn't true of your system, either find another good place and add it to the
test, or x-fail the test.

<rdar://problem/26345962>

llvm-svn: 270014