Error handling mechanism

[mosser]

So far, if the compiler encounters an error, it immediately stops and returns the error on stdout. This is efficient but not ideal. We should update the error processing to get a set of encountered errors. That'll be helpful for the end user, as well as the language server, to send a list of diagnostics.

A diagnostic mode (sending errors using JSON format) could also be envisioned, to ease the integration with a language server.