sg: propagate errors when running individual generate commands

[ggilmore]

Before, error codes wouldn't not be properly propagated back to the main process if you ran an individual sg generate target (ex: sg generate buf) This small change fixes that.

Example output:

~/dev/go/src/github.com/sourcegraph/sourcegraph fix-error-reporting* ≡ 7s
base ❯ go run ./dev/sg  generate buf                                                                                                                                                                 (base) 16:17:08
❌ running "buf generate": exit status 100: searcher.proto:12:45:method searcher.v1.SearcherService.Search: unknown response type SearchResponsefff
exit status 1

Test plan

Manual testing.