Return diagnostics in case of compiler error for integration with Erlang LS

In case of a compilation error, the error is currently printed out via `rebar_api:info/2` and then `rebar_api:abort/0` is called.
From a Language Server perspective, where the plugin is not invoked from command line, but via the Erlang API:

```
rebar3:run(State, ["erl_subgraph_compile", "-f", Filename]).
```

It would be much more convenient to obtain the compiler errors in raw format, so they can be published as [diagnostics](https://microsoft.github.io/language-server-protocol/specifications/specification-current/#diagnostic).

Also see the [els_compiler_diagnostics](https://github.com/erlang-ls/erlang_ls/blob/master/src/els_compiler_diagnostics.erl) module to see how compiler results are currently converted to diagnostics in Erlang LS.