Respect the dbt exit codes

[Bilbottom]

Note

This is an adjusted version of #34, originally raised by @alexmaras.

dbt commands will return exit codes 0, 1, or 2 depending on the run result, but these were being masked by this shim (see #36).

This PR updates the shim to respect the dbt exit codes following the dbt docs:

• https://docs.getdbt.com/reference/programmatic-invocations#dbtrunnerresult

[alexmaras]

Thanks for this - this looks great.