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/function/log.go

@@ -25,7 +25,6 @@ import (
 
 	"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/logdb"
 	"github.com/fission/fission/pkg/fission-cli/util"
 )
@@ -35,14 +34,18 @@ type LogSubCommand struct {
 }
 
 func Log(flags cli.Input) error {
+	c, err := util.GetServer(flags)
+	if err != nil {
+		return err
+	}
 	opts := LogSubCommand{
-		client: cmd.GetServer(flags),
+		client: c,
 	}
 	return opts.do(flags)
 }
 
 func (opts *LogSubCommand) do(flags cli.Input) error {
-	m, err := cmd.GetMetadata("name", "fnNamespace", flags)
+	m, err := util.GetMetadata("name", "fnNamespace", flags)
 	if err != nil {
 		return err
 	}
@@ -66,8 +69,13 @@ func (opts *LogSubCommand) do(flags cli.Input) error {
 		return errors.Wrap(err, "error getting function")
 	}
 
+	server, err := util.GetApplicationUrl("application=fission-api")
+	if err != nil {
+		return err
+	}
+
 	// request the controller to establish a proxy server to the database.
-	logDB, err := logdb.GetLogDB(dbType, util.GetServerUrl())
+	logDB, err := logdb.GetLogDB(dbType, server)
 	if err != nil {
 		return errors.New("failed to connect log database")
 	}