[A11y][Screenreader] Provide updates for IIIF Manifest selection/upload

[Dananji]

Description

When using the timeliner with a screen-reader it was observed that the timeliner doesn't provide adequate information to the screen-reader the success/error status when uploading a IIIF Manifest/using a URL of a IIIF Manifest.

Screen reader/keyboard a11y observation	Impact and/or possible solutions
When the user imports a file in the Local file tab successfully, the user is taken to the timeliner view ❌	The screen reader doesn't announce the success and eventual navigation to the timeliner view
When the user imports a file in the Local file tab unsuccessfully, an error message is displayed ❌	The screen reader doesn't read the error message, and this could be provided using a aria-live region for status updates

Part of avalonmediasystem/avalon#6435.

Done Looks Like

• On erroneous file uploads, the scree reader reads the error message
• On successful file uploads/selection, the screen reader provide updates on the home page -> timeline view page navigation