Input validation and error messages

[mplaine]

Problem 1: Currently, input image dimensions are only validated on the server side.
Solution 1: Validate input image dimensions also on the client side. For example, see https://makitweb.com/how-to-validate-image-width-and-height-before-upload-using-javascript/

Problem 2: In addition, error details sent as a response from the server are NOT shown to the user in the user interface, but only on the console.
Solution 2: Show error details also to the user in the user interface as part of the generic error message.