API rejects multiple role: "user" messages in a row

### Description of the bug:

If you submit a prompt like this to Gemini Pro:

```
{
contents: [
{ role: "user", ...}
{ role: "user", ....}
]
}
```

The API will respond that the last message must come from user.

This occurred when I was translating my GPT role: "system" messages to be role: "user". (But GPT also doesn't reject multiple user messages in a row)

### Actual vs expected behavior:

1. The error message should be accurate
2. The API shouldn't reject two user messages in a row

### Any other information you'd like to share?

_No response_