No error return if Command::Exec execution fails in dockerman

[marmistrz]

To reproduce: take the following snapshot of gumpi: https://github.com/golemfactory/gumpi/tree/4537ac1aa3cb10de6e77fe7ac2a685fda891a2a2

Modify examples/heat_solver.toml: s/--initial/foobar/

Execute: (assuming that the hub is running on 127.0.0.1, with 6 cores available on the machine)

RUST_LOG=gumpi=debug,gu_client=debug cargo run -- --hub 127.0.0.1:61622 --job examples/heat_solver.toml -n 6 --noclean

The error! statement should be executed: https://github.com/golemfactory/gumpi/blob/4537ac1aa3cb10de6e77fe7ac2a685fda891a2a2/src/session/mpi.rs#L264
but in fact the and_then branch is taken: https://github.com/golemfactory/gumpi/blob/4537ac1aa3cb10de6e77fe7ac2a685fda891a2a2/src/session/mpi.rs#L280

This is due to the fact that do_exec in dockerman apparently doesn't handle the return exit code:
https://github.com/golemfactory/golem-unlimited/blob/release/0.2/gu-provider/src/dockerman.rs#L84-L115