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/jrt/jrt-fatals.c

@@ -37,11 +37,6 @@ jerry_fatal (jerry_fatal_code_t code) /**< status code */
       JERRY_ERROR_MSG ("Error: ERR_OUT_OF_MEMORY\n");
       break;
     }
-    case ERR_SYSCALL:
-    {
-      /* print nothing as it may invoke syscall recursively */
-      break;
-    }
     case ERR_REF_COUNT_LIMIT:
     {
       JERRY_ERROR_MSG ("Error: ERR_REF_COUNT_LIMIT\n");