Improve screenshot applet error handling while submitting the screenshot

When the screenshot applet uploads the image it issues a POST request to JIRA server, but does not wait for and analyze the response. As a result, in case the upload fails for some reason, the user has no clue that anything went wrong (unless the network communication fails, but this is a marginal case) and why it went wrong. This might be changed by making the applet wait for response from server, analyze it and display a meaningful message to the user. This may also involve changing the server side code to return easy-to-analyze responses to the applet (as opposed to HTML markup).