Various U/X improvements for helmfile apply (#586)

* Various U/X improvements for `helmfile apply`

This improves the U/X of `helmfile apply`, by allowing you to selectively apply sub-helmfiles.
When you have two or more sub-helmfiles processed, typing `n` to cancel the first doesn't automatically stop the whole helmfile execution.
Instead, it proceeds by diffing the next sub-helmfile, and asks you to apply it, which should be what the user would expect.

To support this workflow, I have suppressed useless exec logs, correct exit status when diff exists in sub-helmfiles but not in the parent helmfile, and made the final error message emitted by helmfile better.

More concretely, this moves more output from `helm` to STDERR and the `debug` log-level.

The overall output from `helmfile` should be a bit more cleaner especially for `apply`, `sync`, `diff` and perhaps other `helmfile` sub-commands, too.

For example, when one of release failed, `helmfile`'s final error message now includes the error message from the failed `helm` execution, like seen in the last line:

```
List of updated releases :
RELEASE   CHART          VERSION
envoy     stable/envoy     1.5.0

List of releases in error :
RELEASE
envoy2
in ./helmfile.yaml: in .helmfiles[0]: in /Users/c-ykuoka/helmfile/helmfile.1.yaml: failed processing release envoy2: helm exited with status 1:
  Error: UPGRADE FAILED: "envoy2" has no deployed releases
```

This way you can better understand what caused helmfile to finally fail.

`helmfile` has been streaminig a lot of stdout and stderr contents from the `helm` commands regardless of the helmfile's log-level. It has been suppressed by default and moved to the `debug` log-level.
You will see that it helps you focus on what was the cause of a failure.

While working on the above, I found an another bug that made `--detailed-exitcode` useless in some case.
That is, `helmfile diff --detailed-exitcode`, when any diff existed only in sub-helmfiles, has been returning an exit code of `1`. It should return `2` when any release had diff and no release had an error, regardless of the target is a sub-helmfile or a parent helmfile. Why? Because that's what `--detailed-exitcode` meant for!

After this PR gets merged, `helmfile diff --detailed-exitcode`  propery return exit code `2` in such cases.

Fixes #543
Resolves #540

helmexec/exec.go

@@ -6,7 +6,6 @@ import (
 	"io/ioutil"
 	"os"
 	"strings"
-
 	"sync"
 
 	"go.uber.org/zap"
@@ -183,7 +182,26 @@ func (helm *execer) DiffRelease(context HelmContext, name, chart string, flags .
 	preArgs := context.GetTillerlessArgs(helm.helmBinary)
 	env := context.getTillerlessEnv()
 	out, err := helm.exec(append(append(preArgs, "diff", "upgrade", "--allow-unreleased", name, chart), flags...), env)
-	helm.write(out)
+	// Do our best to write STDOUT only when diff existed
+	// Unfortunately, this works only when you run helmfile with `--detailed-exitcode`
+	detailedExitcodeEnabled := false
+	for _, f := range flags {
+		if strings.Contains(f, "detailed-exitcode") {
+			detailedExitcodeEnabled = true
+			break
+		}
+	}
+	if detailedExitcodeEnabled {
+		switch e := err.(type) {
+		case ExitError:
+			if e.ExitStatus() == 2 {
+				helm.write(out)
+				return err
+			}
+		}
+	} else {
+		helm.write(out)
+	}
 	return err
 }
 
@@ -227,8 +245,11 @@ func (helm *execer) exec(args []string, env map[string]string) ([]byte, error) {
 	if helm.kubeContext != "" {
 		cmdargs = append(cmdargs, "--kube-context", helm.kubeContext)
 	}
-	helm.logger.Debugf("exec: %s %s", helm.helmBinary, strings.Join(cmdargs, " "))
-	return helm.runner.Execute(helm.helmBinary, cmdargs, env)
+	cmd := fmt.Sprintf("exec: %s %s", helm.helmBinary, strings.Join(cmdargs, " "))
+	helm.logger.Debug(cmd)
+	bytes, err := helm.runner.Execute(helm.helmBinary, cmdargs, env)
+	helm.logger.Debugf("%s: %s", cmd, bytes)
+	return bytes, err
 }
 
 func (helm *execer) info(out []byte) {