Make exec start return proper error codes

Exec start was sending HTTP 500 for every error.

Fixed an error where pausing a container and then calling exec start
caused the daemon to freeze.

Updated API docs which incorrectly showed that a successful exec start
was an HTTP 201, in reality it is HTTP 200.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

docs/reference/api/docker_remote_api.md

@@ -90,6 +90,7 @@ list of DNS options to be used in the container.
 * `GET /events` now includes a `timenano` field, in addition to the existing `time` field.
 * `GET /info` now lists engine version information.
 * `GET /containers/json` will return `ImageID` of the image used by container.
+* `POST /exec/(name)/start` will now return an HTTP 409 when the container is either stopped or paused.
 
 ### v1.20 API changes
 

docs/reference/api/docker_remote_api_v1.15.md

@@ -1677,7 +1677,7 @@ Json Parameters:
 
 Status Codes:
 
--   **201** – no error
+-   **200** – no error
 -   **404** – no such exec instance
 
     **Stream details**:

docs/reference/api/docker_remote_api_v1.16.md

@@ -1638,7 +1638,7 @@ Json Parameters:
 
 Status Codes:
 
--   **201** – no error
+-   **200** – no error
 -   **404** – no such exec instance
 
     **Stream details**:

docs/reference/api/docker_remote_api_v1.17.md

@@ -1801,7 +1801,7 @@ Json Parameters:
 
 Status Codes:
 
--   **201** – no error
+-   **200** – no error
 -   **404** – no such exec instance
 
     **Stream details**:

docs/reference/api/docker_remote_api_v1.18.md

@@ -1922,7 +1922,7 @@ Json Parameters:
 
 Status Codes:
 
--   **201** – no error
+-   **200** – no error
 -   **404** – no such exec instance
 
     **Stream details**:

docs/reference/api/docker_remote_api_v1.19.md

@@ -1984,7 +1984,7 @@ Json Parameters:
 
 Status Codes:
 
--   **201** – no error
+-   **200** – no error
 -   **404** – no such exec instance
 
     **Stream details**:

docs/reference/api/docker_remote_api_v1.20.md

@@ -2126,7 +2126,7 @@ Json Parameters:
 
 Status Codes:
 
--   **201** – no error
+-   **200** – no error
 -   **404** – no such exec instance
 
     **Stream details**:

docs/reference/api/docker_remote_api_v1.21.md

@@ -2226,8 +2226,9 @@ Json Parameters:
 
 Status Codes:
 
--   **201** – no error
+-   **200** – no error
 -   **404** – no such exec instance
+-   **409** - container is stopped or paused
 
     **Stream details**:
     Similar to the stream behavior of `POST /container/(id)/attach` API