backend/s3: Fail fast multipart upload

When a part upload request fails error is returned and gCtx is cancelled.
This does not prevent from other parts being tried.
They immediately fail due to a canceled context, but are retried by rclone anyway...

Example AWS debug output

```
-----------------------------------------------------
2020/02/11 14:12:17 DEBUG: Retrying Request s3/UploadPart, attempt 4
2020/02/11 14:12:17 DEBUG: Request s3/UploadPart Details:
---[ REQUEST POST-SIGN ]-----------------------------
PUT /backuptest-rclone/huge/file.db?partNumber=11&uploadId=190939b4-3c43-4b98-ac11-92303e3f11b0 HTTP/1.1
Host: 192.168.100.99:9000
User-Agent: aws-sdk-go/1.23.8 (go1.13.1; linux; amd64)
Content-Length: 5242880
Authorization: AWS4-HMAC-SHA256 Credential=miniouser/20200211/us-east-1/s3/aws4_request, SignedHeaders=content-length;content-md5;expect;host;x-amz-content-sha256;x-amz-date, Signature=3fc03a01f651cec09b05290459e9ceb26db9a8aa00c4e1b16e8cf5617eb81da8
Content-Md5: XzY+DlipXwbL6bvGYsXftg==
Expect: 100-Continue
X-Amz-Content-Sha256: c036cbb7553a909f8b8877d4461924307f27ecb66cff928eeeafd569c3887e29
X-Amz-Date: 20200211T131217Z
Accept-Encoding: gzip

-----------------------------------------------------
http://192.168.100.99:9000/backuptest-rclone/huge/file.db?partNumber=11&uploadId=190939b4-3c43-4b98-ac11-92303e3f11b0
2020/02/11 14:12:17 DEBUG: Response s3/UploadPart Details:
---[ RESPONSE ]--------------------------------------
HTTP/1.1 500 InternalServerError
Content-Length: 0
-----------------------------------------------------
UploadPartWithContext() error InternalError: We encountered an internal error. Please try again
	status code: 500, request id: , host id:

2020/02/11 14:12:18 DEBUG ERROR: Request s3/UploadPart:
---[ REQUEST DUMP ERROR ]-----------------------------
context canceled
------------------------------------------------------
UploadPartWithContext() error RequestCanceled: request context canceled
caused by: context canceled
2020/02/11 14:12:20 DEBUG ERROR: Request s3/UploadPart:
---[ REQUEST DUMP ERROR ]-----------------------------
context canceled
------------------------------------------------------
UploadPartWithContext() error RequestCanceled: request context canceled
caused by: context canceled
2020/02/11 14:12:22 DEBUG ERROR: Request s3/UploadPart:
---[ REQUEST DUMP ERROR ]-----------------------------
context canceled
------------------------------------------------------
UploadPartWithContext() error RequestCanceled: request context canceled
caused by: context canceled
```

This adds a fail fast behaviour in case the context was cancelled.

Author: mmatczuk

backend/s3/s3.go

@@ -2117,6 +2117,12 @@ func (o *Object) uploadMultipart(ctx context.Context, req *s3.PutObjectInput, si
 			buf = make([]byte, partSize)
 		}
 
+		// Fail fast, in case an errgroup managed function returns an error
+		// gCtx is cancelled. There is no point in uploading all the other parts.
+		if gCtx.Err() != nil {
+			break
+		}
+
 		// Read the chunk
 		var n int
 		n, err = readers.ReadFill(in, buf) // this can never return 0, nil