Errors from start command are discarded causing panics or misleading output

[istyf]

When runBin calls startCmd to start the newly built application, it discards any returned error and proceeds.

On linux this renders misleading output that the process was started even though it wasn't, but on unix/mac and windows the returned cmd is nil (it probably should be on linux as well) causing a panic when air tries to dereference its pid.