Checking the number of bytes written to Mongo GridFS (#889)

* Checking the number of bytes written to Mongo GridFS

* adding missing import

* formatting imports

pkg/storage/mongo/saver.go

@@ -2,6 +2,7 @@ package mongo
 
 import (
 	"context"
+	"fmt"
 	"io"
 
 	"github.com/gomods/athens/pkg/errors"
@@ -23,10 +24,14 @@ func (s *ModuleStore) Save(ctx context.Context, module, version string, mod []by
 	}
 	defer f.Close()
 
-	_, err = io.Copy(f, zip) // check number of bytes written?
+	numBytesWritten, err := io.Copy(f, zip)
 	if err != nil {
 		return errors.E(op, err, errors.M(module), errors.V(version))
 	}
+	if numBytesWritten <= 0 {
+		e := fmt.Errorf("copied %d bytes to Mongo GridFS", numBytesWritten)
+		return errors.E(op, e, errors.M(module), errors.V(version))
+	}
 
 	m := &storage.Module{
 		Module:  module,