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/logdb/influxdb.go

@@ -30,7 +30,6 @@ import (
 	"github.com/pkg/errors"
 
 	ferror "github.com/fission/fission/pkg/error"
-	"github.com/fission/fission/pkg/fission-cli/log"
 )
 
 const (
@@ -84,7 +83,7 @@ func (influx InfluxDB) GetLogs(filter LogFilter) ([]LogEntry, error) {
 	logEntries := []LogEntry{}
 	response, err := influx.query(query)
 	if err != nil {
-		return logEntries, err
+		return nil, err
 	}
 	for _, r := range response.Results {
 		for _, series := range r.Series {
@@ -108,11 +107,11 @@ func (influx InfluxDB) GetLogs(filter LogFilter) ([]LogEntry, error) {
 			for _, row := range series.Values {
 				t, err := time.Parse(time.RFC3339, row[0].(string))
 				if err != nil {
-					log.Fatal(err)
+					return nil, err
 				}
 				seqNum, err := strconv.Atoi(row[seq].(string))
 				if err != nil {
-					return logEntries, err
+					return nil, err
 				}
 				entry := LogEntry{
 					//The attributes of the LogEntry are selected as relative to their position in InfluxDB's line protocol response