api: Sent ErrPreconditionFailed on If-Match failure (#2009)

* api: Sent ErrPreconditionFailed on If-Match failure

ref:
http://docs.aws.amazon.com/AmazonS3/latest/API/ErrorResponses.html#ErrorCodeList

* tests: Added functional tests for GetObject w/ If-Match headers set

* tests: Used verifyError to simplify errorCode and description matching on error

api-errors.go

@@ -74,6 +74,7 @@ const (
 	ErrNoSuchKey
 	ErrNoSuchUpload
 	ErrNotImplemented
+	ErrPreconditionFailed
 	ErrRequestTimeTooSkewed
 	ErrSignatureDoesNotMatch
 	ErrMethodNotAllowed
@@ -255,6 +256,11 @@ var errorCodeResponse = map[APIErrorCode]APIError{
 		Description:    "A header you provided implies functionality that is not implemented.",
 		HTTPStatusCode: http.StatusNotImplemented,
 	},
+	ErrPreconditionFailed: {
+		Code:           "PreconditionFailed",
+		Description:    "At least one of the preconditions you specified did not hold.",
+		HTTPStatusCode: http.StatusPreconditionFailed,
+	},
 	ErrRequestTimeTooSkewed: {
 		Code:           "RequestTimeTooSkewed",
 		Description:    "The difference between the request time and the server's time is too large.",