Fixed the bug and modified the config testing data for exec plugin

internal/config/testdata/telegraf-agent.toml

@@ -107,7 +107,7 @@
 # Read flattened metrics from one or more commands that output JSON to stdout
 [[inputs.exec]]
   # the command to run
-  command = "/usr/bin/mycollector --foo=bar"
+  commands = ["/usr/bin/mycollector --foo=bar"]
   name_suffix = "_mycollector"
 
 # Read metrics of haproxy, via socket or csv stats page

plugins/inputs/tail/tail.go

@@ -89,7 +89,7 @@ func (t *Tail) Start() error {
 	c := NewConfig(t.Files, t.Separator, t.Tags, t.Templates)
 	c.WithDefaults()
 	if err := c.Validate(); err != nil {
-		return fmt.Errorf("Tail input configuration is error! ", err.Error())
+		return fmt.Errorf("Tail input configuration is error: %s ", err.Error())
 	}
 	t.config = c
 
@@ -111,7 +111,7 @@ func (t *Tail) Start() error {
 	for i, fileName := range t.Files {
 		t.tailPointers[i], err = t.tailFile(fileName)
 		if err != nil {
-			fmt.Errorf("Can not open the file: %s to tail", fileName)
+			return fmt.Errorf("Can not open the file: %s to tail", fileName)
 		} else {
 			t.logger.Printf("Openning the file: %s to tail", fileName)
 		}