target, breakpoints: improve error handling

handle_bp_command_set() showed the error message
"Failure setting breakpoint, the same address(IVA) is already used"
on any error returned from (xxx_)breakpoint_add().
Paradoxically breakpoint_add() returned ERROR_OK if it detected
duplicated bp address.
context_breakpoint_add() and hybrid_breakpoint_add() returned -1
instead of OpenOCD compatible error if they detected duplicity.

Introduce ERROR_TARGET_DUPLICATE_BREAKPOINT
Unify error handling to LOG_ERROR() any error in (xxx_)breakpoint_add()
Remove misleading error messages from handle_bp_command_set()
handle_bp_command_set() returns error if the target does not implement
add_context_breakpoint or add_hybrid_breakpoint.

Change-Id: If17dfad1756d82a77028ebdc4b305f9c8e1365ba
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4871
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>