Error page responsiveness, responsive sidebar updated

[adsingh14]

Changes

1. Fix error pages styles and turn into responsive pages.
2. Responsive sidebar style updated.
3. Minor change in responsive book page when open on mobile screen less than 425px width.

Context

There are two similar issues (#1316 and #1522) and nobody has reviewed those changes. Their, I've fixed those issues along with sidebar (which I didn't fix in my previous commit). Now, error pages are looking better and responsive. I've utilised errors.scss.

[adsingh14]

Hi @peff ! I've fixed some pages UI and sidebar UX. Please have a look at it.

[peff]

Minor change in responsive book page when open on mobile.

I had trouble reviewing this part, because I didn't know exactly what you were trying to fix. From looking at the result and at #1650, it looks like this addresses the issue where scrolling down in a long page and revealing the sidebar should show the top of the sidebar content. And it seems to improve that for me (using firefox for android).

[adsingh14]

I had trouble reviewing this part, because I didn't know exactly what you were trying to fix.

That's a patch for smaller screens (425px or less). When we open the Book page, the header misaligned the layout (top most part of book with images).