Add gdb_report_register_access_error command

This replaces the earlier mechanism which would propagate errors only
for targets that decided they wanted to. It was suggested by Matthias
Welwarsky from the OpenOCD team.

Change-Id: Ibe8e97644abb47aff26d74b8280377d42615a4d3

src/target/riscv/riscv.c

@@ -247,7 +247,6 @@ static int riscv_init_target(struct command_context *cmd_ctx,
 		struct target *target)
 {
 	LOG_DEBUG("riscv_init_target()");
-	target->propagate_register_errors = true;
 	target->arch_info = calloc(1, sizeof(riscv_info_t));
 	if (!target->arch_info)
 		return ERROR_FAIL;