Make CLI functions return error instead of fatal out (#1379)

Before this PR, CLI functions fatal out when encountering error
instead of returning it. Such behavior makes it hard to reuse 
the functions nor writing unit tests. This PR aims to make functions 
return errors instead of error out.

pkg/fission-cli/cmd/spec/validate.go

@@ -18,19 +18,17 @@ package spec
 
 import (
 	"bytes"
-	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strings"
 
 	"github.com/hashicorp/go-multierror"
+	"github.com/pkg/errors"
 
 	fv1 "github.com/fission/fission/pkg/apis/fission.io/v1"
 	"github.com/fission/fission/pkg/controller/client"
 	"github.com/fission/fission/pkg/fission-cli/cliwrapper/cli"
-	"github.com/fission/fission/pkg/fission-cli/cmd"
-	"github.com/fission/fission/pkg/fission-cli/log"
 	"github.com/fission/fission/pkg/fission-cli/util"
 )
 
@@ -41,8 +39,12 @@ type ValidateSubCommand struct {
 // Validate parses a set of specs and checks for references to
 // resources that don't exist.
 func Validate(flags cli.Input) error {
+	c, err := util.GetServer(flags)
+	if err != nil {
+		return err
+	}
 	opts := &ValidateSubCommand{
-		client: cmd.GetServer(flags),
+		client: c,
 	}
 	return opts.do(flags)
 }
@@ -54,16 +56,16 @@ func (opts *ValidateSubCommand) do(flags cli.Input) error {
 func (opts *ValidateSubCommand) run(flags cli.Input) error {
 
 	// this will error on parse errors and on duplicates
-	specDir := cmd.GetSpecDir(flags)
+	specDir := util.GetSpecDir(flags)
 	fr, err := ReadSpecs(specDir)
-	util.CheckErr(err, "read specs")
+	if err != nil {
+		return errors.Wrap(err, "error reading specs")
+	}
 
 	// this does the rest of the checks, like dangling refs
 	err = fr.Validate(flags)
 	if err != nil {
-		fmt.Printf("Error validating specs: %v", err)
-
-		return nil
+		return errors.Wrap(err, "error validating specs")
 	}
 
 	return nil
@@ -75,8 +77,8 @@ func ReadSpecs(specDir string) (*FissionResources, error) {
 
 	// make sure spec directory exists before continue
 	if _, err := os.Stat(specDir); os.IsNotExist(err) {
-		log.Fatal(fmt.Sprintf("Spec directory %v doesn't exist. "+
-			"Please check directory path or run \"fission spec init\" to create it.", specDir))
+		return nil, errors.Errorf("Spec directory %v doesn't exist. "+
+			"Please check directory path or run \"fission spec init\" to create it.", specDir)
 	}
 
 	fr := FissionResources{