Exit with status 1 on Cobra problems like a missing required flag

[brandur]

Here, exit with status 1 from the River CLI when a problem intrinsic to
Cobra occurred like the user entered an unknown command, or didn't pass
required flag.

Cobra is a little odd in its behavior in that it already prints such a
problem to the terminal, but also returns an error with the same
information. This change looks for the presence of an error and exits
with status 1 if one occurred, but doesn't double print any additional
information about it.

Fixes #362.

[brandur]

Tested locally as well:

$ go run . migrate-up
Error: required flag(s) "database-url" not set
Usage:
  river migrate-up [flags]

Flags:
      --database-url postgres://...   URL of the database to migrate (should look like postgres://...
      --dry-run                       print information on migrations, but don't apply them
  -h, --help                          help for migrate-up
      --max-steps int                 maximum number of steps to migrate
      --show-sql                      show SQL of each migration
      --target-version int            target version to migrate to (final state includes this version, but none after it)

Global Flags:
      --debug     output maximum logging verbosity (debug level)
  -v, --verbose   output additional logging verbosity (info level)

exit status 1

$ echo $?
1

[brandur]

Thanks!