Catch E486 to prevent error after removing silent navigation

[myii]

This follows-on from issue #33, which resulted in commit b425bb4.

Unfortunately, one side-effect of removing silent! is that E486: Pattern not found: <pattern> is now seen as an error in WordNavigation. Also requires the extra key press to get rid of the error.

Credit for this resolution is:

• https://github.com/tweekmonster/braceless.vim/blob/3928fe18fb7c8561beed6a945622fd985a8e638b/autoload/braceless/python/format.vim#L120-L126

Finally, this isn't perfect, in that the E486 message is no longer displayed after hitting n or N -- instead, it shows search hit BOTTOM, continuing at TOP (or vice versa). But the meaning is still correct and this avoids the ugly error.

[myii]

Just added another commit, which simulates the E486 as a warning instead. In a general sense, that brings the functionality in line with standard Vim.

[lfv89]

I was going to work on this today! 😄

Thanks again @myii

[myii]

You're welcome.