Catch exceptions for render/entity failures

Currently, if an exception is thrown when generating a block, a huge horrible fatal error is generated. Really we don't need to have anything quite that exotic. Why don't we put a try/catch around the block generation and not render the block (and emit a regular _drupal_log_error() or something) so that the administrator gets the information, but we don't deliver a full fatal error for every page where the block is to be rendered.