Return 400 instead of 500 if generic backend is written to without data.

Fixes #825

CHANGELOG.md

@@ -97,6 +97,8 @@ generate them, leading to client errors.
  * secret/generic: Validate given duration at write time, not just read time;
    if stored durations are not parseable, return a warning and the default
    duration rather than an error [GH-718]
+ * secret/generic: Return 400 instead of 500 when `generic` backend is written
+   to with no data fields [GH-825]
  * secret/postgresql: Revoke permissions before dropping a user or revocation
    may fail [GH-699]
 

vault/logical_passthrough.go

@@ -149,7 +149,7 @@ func (b *PassthroughBackend) handleWrite(
 	req *logical.Request, data *framework.FieldData) (*logical.Response, error) {
 	// Check that some fields are given
 	if len(req.Data) == 0 {
-		return nil, fmt.Errorf("missing data fields")
+		return logical.ErrorResponse("missing data fields"), nil
 	}
 
 	// Check if there is a ttl key; verify parseability if so