Add additional error fallbacks during rendering.

[copybara-service]

Add additional error fallbacks during rendering.

• Avoid calling repr in the renderer last-resort fallback, since
  this is user-defined and can still throw an exception for invalid
  objects.
• Catch errors while rendering deferred parts, to avoid breaking the
  IPython display integration if a deferred part raises an exception.
  (This can happen due to bugs in array rendering adapters, for instance.)
  Before, errors in deferred parts could bubble out to IPython, which would
  fall back to text rendering, even though a partial HTML output was already
  shown.
• Add tests for custom handlers and error checking logic.