server : return HTTP 400 if prompt exceeds context length (#16486)

In streaming mode when prompt exceeds context length, the server returns
HTTP 200 status code with a JSON error in the body.  This is very
confusing and inconsistent with all other inference engines which return
HTTP 4xx error in this case.

This patch fixes this problem and makes the server return HTTP 400 in
such cases.

tools/server/tests/utils.py

@@ -35,6 +35,12 @@ class ServerResponse:
     body: dict | Any
 
 
+class ServerError(Exception):
+    def __init__(self, code, body):
+        self.code = code
+        self.body = body
+
+
 class ServerProcess:
     # default options
     debug: bool = False
@@ -297,6 +303,8 @@ class ServerProcess:
             response = requests.post(url, headers=headers, json=data, stream=True)
         else:
             raise ValueError(f"Unimplemented method: {method}")
+        if response.status_code != 200:
+            raise ServerError(response.status_code, response.json())
         for line_bytes in response.iter_lines():
             line = line_bytes.decode("utf-8")
             if '[DONE]' in line: