helper/resource: Fix data race in resource.Retry
helper/resource: Fix data race in resource.Retry

The implementation was attempting to capture the error using a scoped
variable reference, but the error value is already exposed via the
return value of `WaitForState()`. Using that instead fixes the data
race.

Fixes the following data race:

```
==================
WARNING: DATA RACE
Read by goroutine 74:
  github.com/hashicorp/terraform/helper/resource.Retry()
      /Users/phinze/go/src/github.com/hashicorp/terraform/helper/resource/wait.go:35 +0x284
  github.com/hashicorp/terraform/helper/resource.TestRetry_timeout()
      /Users/phinze/go/src/github.com/hashicorp/terraform/helper/resource/wait_test.go:35 +0x60
  testing.tRunner()
      /private/var/folders/vd/7l9ys5k57l91x63sh28wl_kc0000gn/T/workdir/go/src/testing/testing.go:456 +0xdc

Previous write by goroutine 90:
  github.com/hashicorp/terraform/helper/resource.Retry.func1()
      /Users/phinze/go/src/github.com/hashicorp/terraform/helper/resource/wait.go:20 +0x87
  github.com/hashicorp/terraform/helper/resource.(*StateChangeConf).WaitForState.func1()
      /Users/phinze/go/src/github.com/hashicorp/terraform/helper/resource/state.go:83 +0x284

Goroutine 74 (running) created at:
  testing.RunTests()
      /private/var/folders/vd/7l9ys5k57l91x63sh28wl_kc0000gn/T/workdir/go/src/testing/testing.go:561 +0xaa3
  testing.(*M).Run()
      /private/var/folders/vd/7l9ys5k57l91x63sh28wl_kc0000gn/T/workdir/go/src/testing/testing.go:494 +0xe4
  main.main()
      github.com/hashicorp/terraform/helper/resource/_test/_testmain.go:84 +0x20f

Goroutine 90 (running) created at:
  github.com/hashicorp/terraform/helper/resource.(*StateChangeConf).WaitForState()
      /Users/phinze/go/src/github.com/hashicorp/terraform/helper/resource/state.go:127 +0x283
  github.com/hashicorp/terraform/helper/resource.Retry()
      /Users/phinze/go/src/github.com/hashicorp/terraform/helper/resource/wait.go:34 +0x276
  github.com/hashicorp/terraform/helper/resource.TestRetry_timeout()
      /Users/phinze/go/src/github.com/hashicorp/terraform/helper/resource/wait_test.go:35 +0x60
  testing.tRunner()
      /private/var/folders/vd/7l9ys5k57l91x63sh28wl_kc0000gn/T/workdir/go/src/testing/testing.go:456 +0xdc
==================
```

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk