* Validate value of `!RELEASE_MODE!`
* Exit with `ERRORLEVEL` set to `1` on errors.
Previously we did not and so apps that use releases and rely on exit
codes to report errors were not doing so (e.g. Livebook).
* Rename rpc node name from `rem-(...)` to `rpc-(...)`
* Update generated `rel/env.bat.exs`