softmmu: Provide a clue as to why device tree loading failed

fdt_open_into() obligingly returns an error code in case the operation
failed. So be obliging as well and use it in the error message.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220116114649.40859-1-shentey@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

softmmu/device_tree.c

@@ -60,7 +60,8 @@ void *create_device_tree(int *sizep)
     }
     ret = fdt_open_into(fdt, fdt, *sizep);
     if (ret) {
-        error_report("Unable to copy device tree in memory");
+        error_report("%s: Unable to copy device tree into memory: %s",
+                     __func__, fdt_strerror(ret));
         exit(1);
     }
 
@@ -104,7 +105,8 @@ void *load_device_tree(const char *filename_path, int *sizep)
 
     ret = fdt_open_into(fdt, fdt, dt_size);
     if (ret) {
-        error_report("Unable to copy device tree in memory");
+        error_report("%s: Unable to copy device tree into memory: %s",
+                     __func__, fdt_strerror(ret));
         goto fail;
     }