Now MinioAPI handles Content-MD5 set during putObject()

- This change also facilitates proper error response
    in case of md5 mismatch or corruption
  - TODO a test function needs to be implemented

pkg/storage/storage_errors.go

@@ -47,6 +47,13 @@ type ImplementationError struct {
 	Err    error
 }
 
+// DigestError - Generic Md5 error
+type DigestError struct {
+	Bucket string
+	Key    string
+	Md5    string
+}
+
 /// Bucket related errors
 
 // BucketPolicyNotFound - missing bucket policy
@@ -72,6 +79,12 @@ type ObjectExists GenericObjectError
 // ObjectNameInvalid - object name provided is invalid
 type ObjectNameInvalid GenericObjectError
 
+// BadDigest - md5 mismatch from data received
+type BadDigest DigestError
+
+// InvalidDigest - md5 in request header invalid
+type InvalidDigest DigestError
+
 // Return string an error formatted as the given text
 func (e ImplementationError) Error() string {
 	error := ""
@@ -138,3 +151,13 @@ func (e ObjectNameInvalid) Error() string {
 func (e BackendCorrupted) Error() string {
 	return "Backend corrupted: " + e.Path
 }
+
+// Return string an error formatted as the given text
+func (e BadDigest) Error() string {
+	return "Md5 provided " + e.Md5 + " mismatches for: " + e.Bucket + "#" + e.Key
+}
+
+// Return string an error formatted as the given text
+func (e InvalidDigest) Error() string {
+	return "Md5 provided " + e.Md5 + " is invalid"
+}