cmd/river: Exit code 0 on flag error

[vfaronov]

When I run:

$ go run github.com/riverqueue/river/cmd/river@latest migrate-up

I get:

Error: required flag(s) "database-url" not set
Usage:
[...]

But the exit code is 0, which means success:

$ echo $?
0

The command should exit with a non-zero code on error.

[brandur]

Thanks. Fixed in #363.