Only a handful of critical errors are present in the codebase.
They all occur during server startup (in `app.StartServer()`).
Currently, when one of these critical error occurs, it is simpled
mentionned in the logs – then the error is discarded, and the app
attempts to continue the execution (and probably fails pretty quickly in
a weird way).
Rather than continuing operations in an unknow state, these errors should
trigger a clean exit.
This commit rewrites critical startup errors to be correctly
propagated, logged, and then terminate the command execution.
Additionnaly, it makes the server return a proper error code to the
shell.
