Fix default toast message for failed uploads with HTTP/2

Fix default toast message for failed uploads with HTTP/2

When an upload fails a toast is shown with either a specific message or
just the textual part of the HTTP error code (which comes from the
upload failure handler and set by "jQuery.ajax()". However, if there
is neither a message nor an error then the toast will show the default
message from the Toastify-js library, which is an undescriptive "Hi
there!".

When HTTP/2 is used Chromium does not provide the textual part of the
HTTP error code, so when an upload fails the toast can receive an empty
message and thus just show "Hi there!". Now an explicit message is
provided as a fallback to prevent that.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>