Don't let failures to load the null WebView propagate.

We load the null WebView if the real WebView package is not available to
support devices that aren't intended to have a WebView, but this results
in confusing log messages if the real WebView is supposed to be present
but is broken/unloadable for some reason. Since the null WebView is in
the system classpath on devices that should use it, swallow any failures
to load it on the assumption that this is actually just a failure of the
real WebView to be loaded.

Change-Id: I15820da3517daffde6169343c648a4486c0309d7