bucketExists: Use a bool to indicate if a bucket exists or not

This is a breaking API change. bucketExists API will require
a callback with this signature cb(err, bool). This is a better
way to indicate whether a bucket exists or not.

docs/API.md

@@ -162,7 +162,7 @@ minioClient.listBuckets(function(err, buckets) {
 ```
 
 <a name="bucketExists"></a>
-#### bucketExists(bucketName[, callback])
+#### bucketExists(bucketName, callback)
 
 Checks if a bucket exists.
 
@@ -173,19 +173,19 @@ __Parameters__
 | Param  | Type  | Description  |
 |---|---|---|
 | `bucketName`  |  _string_ | Name of the bucket.  |
-| `callback(err)`  | _function_  | `err` is `null` if the bucket exists. `err.code` is `NoSuchBucket` in case the bucket does not exist. If no callback is passed, a `Promise` is returned. |
+| `callback(err, exists)`  | _function_  | `exists` is a boolean which indicates whether `bucketName` exists or not. `err` is set when an error occurs during the operation. |
 
 __Example__
 
 
 ```js
-minioClient.bucketExists('mybucket', function(err) {
+minioClient.bucketExists('mybucket', function(err, exists) {
   if (err) {
-     if (err.code == 'NoSuchBucket') return console.log("bucket does not exist.")
-     return console.log(err)
+    return console.log(err)
+  }
+  if (exists) {
+    return console.log('Bucket exists.')
   }
-  // if err is null it indicates that the bucket exists.
-  console.log('Bucket exists.')
 })
 ```
 

examples/bucket-exists.js

@@ -26,9 +26,11 @@ var s3Client = new Minio.Client({
   secretKey: 'YOUR-SECRETACCESSKEY'
 })
 
-s3Client.bucketExists('my-bucketname', function(e) {
-  if (e) {
-    return console.log(e)
+s3Client.bucketExists('my-bucketname', function(err, exists) {
+  if (err) {
+    return console.log(err)
+  }
+  if (exists) {
+      console.log("Bucket exists.")
   }
-  console.log("True")
 })

src/main/minio.js

@@ -647,7 +647,13 @@ export class Client {
       throw new TypeError('callback should be of type "function"')
     }
     var method = 'HEAD'
-    this.makeRequest({method, bucketName}, '', 200, '', false, cb)
+    this.makeRequest({method, bucketName}, '', 200, '', false, err => {
+      if (err) {
+        if (err.code == 'NoSuchBucket' || err.code == 'NotFound') return cb(null, false)
+        return cb(err)
+      }
+      cb(null, true)
+    })
   }
 
   // Remove a bucket.