(bug 49241) Handle failures in {{#property}} gracefully.

This catches any exception that ocurr while rendering the property,
and remembers them in the ParserOutput object to show them on
the finished page.

The parser function itself returns an empty string on failure.

Change-Id: I8cf39fd8f34d36dfdd90710c5fbd3337b7059df2
Merge "(bug 49241) Handle failures in {{#property}} gracefully."