api docs: Test sample fixture for nonexistent stream error.

In templates/zerver/api/stream-message.md, we have a sample fixture
for when the target stream does not exist. This commit adds a test
for that sample fixture.

templates/zerver/api/fixtures.json

@@ -238,5 +238,10 @@
     "delete-queue": {
         "msg":"",
         "result":"success"
+    },
+    "nonexistent-stream-error": {
+        "code":"BAD_REQUEST",
+        "msg":"Stream 'nonexistent_stream' does not exist",
+        "result":"error"
     }
 }

templates/zerver/api/remove-subscriptions.md

@@ -89,13 +89,6 @@ A typical successful JSON response may look like:
 
 {generate_code_example|remove-subscriptions|fixture}
 
-A typical JSON response for when you try to unsubscribe from a stream
-that doesn't exist:
+A typical failed JSON response for when the target stream does not exist:
 
-```
-{
-    "msg":"Stream(s) (Denmarkk) do not exist",
-    "code":"BAD_REQUEST",
-    "result":"error"
-}
-```
+{generate_code_example|nonexistent-stream-error|fixture}

templates/zerver/api/stream-message.md

@@ -104,10 +104,4 @@ A typical successful JSON response may look like:
 
 A typical failed JSON response for when the target stream does not exist:
 
-```
-{
-    'code':'BAD_REQUEST',
-    'msg':"Stream 'Denmarkk' does not exist",
-    'result':'error'
-}
-```
+{generate_code_example|nonexistent-stream-error|fixture}

zerver/lib/api_test_helpers.py

@@ -251,6 +251,19 @@ def stream_message(client):
 
     return message_id
 
+def test_nonexistent_stream_error(client):
+    # type: (Client) -> None
+    request = {
+        "type": "stream",
+        "to": "nonexistent_stream",
+        "subject": "Castle",
+        "content": "Something is rotten in the state of Denmark."
+    }
+    result = client.send_message(request)
+
+    fixture = FIXTURES['nonexistent-stream-error']
+    test_against_fixture(result, fixture)
+
 def private_message(client):
     # type: (Client) -> None
 
@@ -394,6 +407,8 @@ def test_messages(client):
     update_message(client, message_id)
     private_message(client)
 
+    test_nonexistent_stream_error(client)
+
 def test_users(client):
     # type: (Client) -> None