I'm using the Stormpath Angular SDK with Spring Boot Stormpath Starter version 2.0.0-okta-rc1. If I enter a bad password, the error message is "Invalid grant" rather than "Invalid username or password.".

You can reproduce this problem using HTTPie. The following will work.
http -f POST localhost:8080/oauth/token grant_type=password username=VALID_USERNAME password=CORRECT_PASSWORD
This does not:
http -f POST localhost:8080/oauth/token grant_type=password username=VALID_USERNAME password=BAD_PASSWORD
HTTP/1.1 400
Cache-Control: no-store, no-cache
Connection: close
Content-Length: 53
Content-Type: application/json;charset=ISO-8859-1
Date: Thu, 01 Jun 2017 14:58:09 GMT
Expires: 0
Pragma: no-cache
Set-Cookie: JSESSIONID=6A12DD8C7656A3BD4ADD9EEE5DBF6B02; Path=/; HttpOnly
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
X-XSS-Protection: 1; mode=block
{
"error": "invalid_request",
"message": "Invalid grant"
}