gdb: connect will now fail if flash autoprobe fails
This stops GDB from launching with an empty memory map,
making gdb load w/flashing fail for no obvious reason.
The error message points in the direction of the gdb-attach
event that can be set up to issue a halt or "reset init"
which will put GDB in a well defined stated upon attach
and thus have a robust flash autoprobe.
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>