refactor: error var missingXXX should have name of the form errFoo

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

README.md

@@ -16,13 +16,32 @@ information and a listing of the available options please take a look at [the do
 **Note: Please update your image config path to `appleboy/drone-ssh` for drone. `plugins/ssh` is no longer maintained.**
 
 ![demo](./images/demo2017.05.10.gif)
-## Build
 
-Build the binary with the following commands:
+## Build or Download a binary
+
+The pre-compiled binaries can be downloaded from [release page](https://github.com/appleboy/drone-ssh/releases). Support the following OS type.
+
+* Windows amd64/386
+* Linux arm/amd64/386
+* Darwin amd64/386
+
+With `Go` installed
 
 ```
-go build
-go test
+$ go get -u -v github.com/appleboy/drone-ssh
+``` 
+
+or build the binary with the following command:
+
+```
+$ export GOOS=linux
+$ export GOARCH=amd64
+$ export CGO_ENABLED=0
+$ export GO111MODULE=on
+
+$ go test -cover ./...
+
+$ go build -v -a -tags netgo -o release/linux/amd64/drone-ssh .
 ```
 
 ## Docker

plugin.go

@@ -14,10 +14,10 @@ import (
 )
 
 var (
-	missingHost          = errors.New("Error: missing server host")
-	missingPasswordOrKey = errors.New("Error: can't connect without a private SSH key or password")
-	commandTimeOut       = errors.New("Error: command timeout")
-	setPasswordandKey    = errors.New("can't set password and key at the same time")
+	errMissingHost          = errors.New("Error: missing server host")
+	errMissingPasswordOrKey = errors.New("Error: can't connect without a private SSH key or password")
+	errCommandTimeOut       = errors.New("Error: command timeout")
+	errSetPasswordandKey    = errors.New("can't set password and key at the same time")
 )
 
 type (
@@ -118,7 +118,7 @@ func (p Plugin) exec(host string, wg *sync.WaitGroup, errChannel chan error) {
 
 		// command time out
 		if !isTimeout {
-			errChannel <- commandTimeOut
+			errChannel <- errCommandTimeOut
 		}
 	}
 
@@ -139,15 +139,15 @@ func (p Plugin) log(host string, message ...interface{}) {
 // Exec executes the plugin.
 func (p Plugin) Exec() error {
 	if len(p.Config.Host) == 0 {
-		return missingHost
+		return errMissingHost
 	}
 
 	if len(p.Config.Key) == 0 && len(p.Config.Password) == 0 && len(p.Config.KeyPath) == 0 {
-		return missingPasswordOrKey
+		return errMissingPasswordOrKey
 	}
 
 	if len(p.Config.Key) != 0 && len(p.Config.Password) != 0 {
-		return setPasswordandKey
+		return errSetPasswordandKey
 	}
 
 	wg := sync.WaitGroup{}

plugin_test.go

@@ -16,7 +16,7 @@ func TestMissingHostOrUser(t *testing.T) {
 	err := plugin.Exec()
 
 	assert.NotNil(t, err)
-	assert.Equal(t, missingHost, err)
+	assert.Equal(t, errMissingHost, err)
 }
 
 func TestMissingKeyOrPassword(t *testing.T) {
@@ -31,7 +31,7 @@ func TestMissingKeyOrPassword(t *testing.T) {
 	err := plugin.Exec()
 
 	assert.NotNil(t, err)
-	assert.Equal(t, missingPasswordOrKey, err)
+	assert.Equal(t, errMissingPasswordOrKey, err)
 }
 
 func TestSetPasswordAndKey(t *testing.T) {
@@ -48,7 +48,7 @@ func TestSetPasswordAndKey(t *testing.T) {
 	err := plugin.Exec()
 
 	assert.NotNil(t, err)
-	assert.Equal(t, setPasswordandKey, err)
+	assert.Equal(t, errSetPasswordandKey, err)
 }
 
 func TestIncorrectPassword(t *testing.T) {