Display field errors from the backend in the metadata modal

The metadata modal's form already has the proper template and a this.formErrors to display errors on each form field. Instead of just showing a generic Request failed with status code 400, we can handle some of the errors and set this.formErrors to the error message to allow the template to show more explicit errors. For example, {"value": ["The value of a numeric metadata should be a valid floating-point number."]} could display the message directly under the field: