Error when running outside of Rebar Project

Should give a proper warning if it's being ran outside of a rebar project.

```
$ bombom
Verifying dependencies...
escript: exception error: no match of right hand side value []
in function rebar3_sbom_prv:do/1 (xyz/_build/default/plugins/rebar3_sbom/src/rebar3_sbom_prv.erl:62)
in call from rebar_core:do/2 (bombom/_build/default/lib/rebar/apps/rebar/src/rebar_core.erl:155)
in call from rebar3:run_aux/2 (bombom/_build/default/lib/rebar/apps/rebar/src/rebar3.erl:212)
in call from bombom:main/1 (bombom/src/bombom.erl:18)
in call from escript:run/2 (escript.erl:906)
in call from escript:start/1 (escript.erl:420)
in call from init:start_it/1
in call from init:start_em/1
```

Reproduce the failure by running `bombom` outside a Rebar project, then read `src/bombom.erl` around line 18 and the reported `rebar3_sbom_prv.erl` line 62. Done means the same invocation gives a proper warning instead of the shown exception.

Written by the indexing model from the issue text.