Crash: rust model panic during getMarkdown()

[bmarty]

Problem

Calling getMarkdown() may cause a crash if the underlying Rust model panics.

Please see the details in

• Crash from RichTextEditor element-hq/element-x-android#1511

Solution

• Catch and track any rust panic during getMarkdown() method (perhaps others too).
• Fall back to get_content_as_plain_text() if getMarkdown() fails.
• Fall back to empty string if get_content_as_plain_text() fails.