Maintenance of fatal code enum (#2670)

- `ERR_SYSCALL` is a legacy that has not been used anymore since
  the removal of jerry-libc. This commit removes it from the port
  API.
- `ERR_DISABLED_BYTE_CODE` is a (relatively) recent addition to the
  fatal code enum. At the time it was added, the documentation has
  not been updated. This commit adds the missing documentation.
  (Plus, it removes a superfluous `JERRY_UNREACHABLE` from after a
  `jerry_fatal (ERR_DISABLED_BYTE_CODE)`.)

Note: As the port API is modified, this is an API-breaking change.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/vm/vm.c

@@ -3112,7 +3112,6 @@ vm_loop (vm_frame_ctx_t *frame_ctx_p) /**< frame context */
           JERRY_ASSERT (VM_OC_GROUP_GET_INDEX (opcode_data) == VM_OC_NONE);
 
           jerry_fatal (ERR_DISABLED_BYTE_CODE);
-          JERRY_UNREACHABLE ();
         }
       }