Plugin crashes on non-existing files

```
$ rebar3 erl_subgraph_compile -f src/foo.erl
===> Expanded command sequence to be run: [{default,app_discovery},
{default,
erl_subgraph_compile}]
===> Provider: {default,app_discovery}
===> Provider: {default,erl_subgraph_compile}
===> Uncaught error in rebar_core. Run with DEBUG=1 to see stacktrace or consult rebar3.crashdump
===> Uncaught error: {case_clause,false}
===> Stack trace to the error location:
[{erl_subgraph_compile_prv,level_order,3,
[{file,"/Users/robert.aloi/git/github/erlang-ls/rebar3_bsp/priv/sample/_build/default/plugins/erl_subgraph_compile/src/erl_subgraph_compile_prv.erl"},
{line,196}]},
{erl_subgraph_compile_prv,resolve_chain,3,
[{file,"/Users/robert.aloi/git/github/erlang-ls/rebar3_bsp/priv/sample/_build/default/plugins/erl_subgraph_compile/src/erl_subgraph_compile_prv.erl"},
{line,186}]},
{erl_subgraph_compile_prv,do,1,
[{file,"/Users/robert.aloi/git/github/erlang-ls/rebar3_bsp/priv/sample/_build/default/plugins/erl_subgraph_compile/src/erl_subgraph_compile_prv.erl"},
{line,50}]},
{rebar_core,do,2,
[{file,"/Users/robert.aloi/git/github/erlang/rebar3/src/rebar_core.erl"},
{line,154}]},
{rebar3,run_aux,2,
[{file,"/Users/robert.aloi/git/github/erlang/rebar3/src/rebar3.erl"},
{line,181}]},
{rebar3,main,1,
[{file,"/Users/robert.aloi/git/github/erlang/rebar3/src/rebar3.erl"},
{line,66}]},
{escript,run,2,[{file,"escript.erl"},{line,758}]},
{escript,start,1,[{file,"escript.erl"},{line,277}]}]
===> When submitting a bug report, please include the output of `rebar3 report "your command"`
```