Render 404 instead of 500 if trying to see snippets of unexisting user

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
 Merge branch 'snippets-page' into 'master'

Render 404 instead of 500 if trying to see snippets of unexisting user

Fixes #1087