Simplify data verification with HashReader. (#5071)

Verify() was being called by caller after the data
has been successfully read after io.EOF. This disconnection
opens a race under concurrent access to such an object.
Verification is not necessary outside of Read() call,
we can simply just do checksum verification right inside
Read() call at io.EOF.

This approach simplifies the usage.

cmd/object-api-errors.go

@@ -93,8 +93,6 @@ func toObjectErr(err error, params ...string) error {
 		err = InsufficientWriteQuorum{}
 	case io.ErrUnexpectedEOF, io.ErrShortWrite:
 		err = IncompleteBody{}
-	case errContentSHA256Mismatch:
-		err = SHA256Mismatch{}
 	}
 	if ok {
 		e.e = err
@@ -103,13 +101,6 @@ func toObjectErr(err error, params ...string) error {
 	return err
 }
 
-// SHA256Mismatch - when content sha256 does not match with what was sent from client.
-type SHA256Mismatch struct{}
-
-func (e SHA256Mismatch) Error() string {
-	return "sha256 computed does not match with what is expected"
-}
-
 // SignatureDoesNotMatch - when content md5 does not match with what was sent from client.
 type SignatureDoesNotMatch struct{}
 
@@ -200,16 +191,6 @@ func (e BucketExists) Error() string {
 	return "Bucket exists: " + e.Bucket
 }
 
-// BadDigest - Content-MD5 you specified did not match what we received.
-type BadDigest struct {
-	ExpectedMD5   string
-	CalculatedMD5 string
-}
-
-func (e BadDigest) Error() string {
-	return "Bad digest: Expected " + e.ExpectedMD5 + " is not valid with what we calculated " + e.CalculatedMD5
-}
-
 // UnsupportedDelimiter - unsupported delimiter.
 type UnsupportedDelimiter struct {
 	Delimiter string