Descriptive error messages when media fails to send.

[pixlwave]

When a message fails to send, the standard text "Messages failed to send" is shown. Regarding videos, along with network issues, the failure could also be because of:

• The encoder failed.
• The media exceeds the homeserver's max upload size

We should add some more reasons for failure to properly communicate what went wrong, possibly falling back to "Messages failed to send" if more than one type of error occurred.

In this instance the video failed to encode, but it isn't obvious: