Alttxt error handling

[NateLanza]

• When an alttxt is generated for an aggregated plot, handle the 500 error from the alttxt API and display an appropriate error message in the element view.
• If an error occurs when the plot isn't aggregated, show a more generic error message, possibly split by error type (404, other 500, etc).
• Look for any other plot configurations that cause API errors and generate error message for them.