Limit memory mode at 100 buckets and return error appropriately

~~~
mc: <ERROR> Failed to create bucket for URL [http://localhost:9000/newbucket-101]. Reason: [You have attempted to create more buckets than allowed.].
~~~

Response from server

pkg/storage/drivers/errors.go

@@ -78,6 +78,9 @@ type BucketExists GenericBucketError
 // BucketNotFound - requested bucket not found
 type BucketNotFound GenericBucketError
 
+// TooManyBuckets - total buckets exceeded
+type TooManyBuckets GenericBucketError
+
 /// Object related errors
 
 // ObjectNotFound - requested object not found
@@ -149,6 +152,11 @@ func (e BucketExists) Error() string {
 	return "Bucket exists: " + e.Bucket
 }
 
+// Return string an error formatted as the given text
+func (e TooManyBuckets) Error() string {
+	return "Bucket limit exceeded beyond 100, cannot create bucket: " + e.Bucket
+}
+
 // Return string an error formatted as the given text
 func (e BucketNotFound) Error() string {
 	return "Bucket not Found: " + e.Bucket