Better error message for long headings

[pcraig3]

When you import a Word document with a really long (paragraph-length) block tagged as a heading, it throws up an exception related to the line length, which originates in the in the model definition for aSection.

This paragraph in the orange box is styled as a Heading 3

This is actually desired behaviour, since we don't ever see headings that are this large. In (probably) every case, these are paragraph elements that have been mistagged as headings.

The problem is that the error message we return to the user doesn't explain the issue very well. It talks about the character length but not about what's the root issue.

This story is about adding an error message that communicates to users what is happening.

What is happening is that they have mistagged paragraph text as heading text. We should tell them some version of this ('this document can't be imported because'), and also indicate where in the document this issue is (either referring to the text itself or the previous heading).

Right now the 400-level error being thrown is a general catch-all error route. Check out this commit for the origin of this: 4981aa4#diff-9438ebabdf1c1d43f10f6eafb9bdcb6459283b95943993031f9660187da51b0fR10

[theresaanna]

Note to self: message Adam for example error message.

[pcraig3]

Let me know if you need an example document for testing this.

[pcraig3]

This is closed since #97 went in.