Mattermost-plugin-github: Provide a more clear error message when the wrong content type is selected for a webhook

[jasonblais]

Repro:

1. Follow these steps to configure the GitHub plugin: https://github.com/mattermost/mattermost-plugin-github/#configuration
   
   
   
   • NOTE: To reproduce the error in this issue, when creating the webhook in GitHub in step 2, do not change the content type to "application/json"
   
   
2. Connect your Mattermost account to GitHub with /github connect.
3. Subscribe a channel to receive notifications with /github subscribe owner/repo.
4. Trigger an event such as opening an issue in the GitHub repository you subscribed.

Observed: No event, with the following error in server logs

2019-06-17T13:35:18.794Z    info    go-plugin/stream.go:15    {"level":"error","msg":"invalid character 'p' looking for beginning of value"}    {"plugin_id": "github", "source": "plugin_stdout"}

Proposed change:

1. Update the server log error message to state GitHub webhook content type should be set to "application/json"
2. Fail the request with a proper status code so that the webhook shows up as failed on GitHub

[niklabh]

Here is a pull request: https://github.com/mattermost/mattermost-plugin-github/pull/121